The new Nikon D5100 sits just above the D3100, a bit below the aging D90, and several steps below the fully-featured and highly customizable Nikon D7000. The D5100 boasts a 16 megapixel image sensor (just like the D7000), shoots 4 frames per second in continuous mode, has HD video capability at 24, 25, and 30 fps, and includes the fully rotating rear LCD screen improved in flexibility from the screen of the D5000.
Generally as the cameras increase in price and capability from the entry level model to the enthusiast pro-sumer model they gain more sophisticated autofocus and exposure metering systems, shoot faster (more frames per second) in continuous shooting mode, have more controls and buttons for changing settings on the camera body, and offer more menu and custom function options.
Sensor and Image Quality: The sensors of the D5100 and D7000 are greatly improved over the older D90 in a couple of ways. The D5100 and D7000 have 16.2 megapixel sensors, where the D90 has 12.3 megapixels. The relatively new D3100 has a 14.2 MP sensor. This increase in resolution allows for more intrusive editing of the files in Photoshop, the ability to crop a picture and still obtain an image with high enough resolution for printing or display, and allows for larger prints. In addition, the improved sensor results in better performance at high ISO settings and in low light, better dynamic range, tonal range, and color sensitivity. Have a look at the dxomark website to compare the sensors.
Exposure Metering: The D5100, while sharing a similar sensor to the D7000, does not have the same advanced metering system. It shares the less sophisticated 420 pixel RGB metering sensor of the D3100 and offers matrix metering, non-adjustable center-weighted, and spot metering modes. This system may be more than sufficient for many users, especially those not intending to adjust their exposure settings and dig into their menus in reaction to complex lighting situations. But if your shooting demands require more precise exposure metering and control over the size of the areas being metered, you need to consider the D7000 or D90. The 2016 pixel RGB metering sensor of the D7000 is also improved compared to the D90, and will result in more accurate metering performance of straightforward and complex lighting scenes and situations. Both these cameras offer matrix metering, center-weighted, and spot metering modes. With center-weighted metering on the D90, you can select the size of the center-weighted area to be a 6, 8, or 10mm center circle, and the D7000 adds a 13mm circle option.
Autofocus: The autofocus systems of the D3100, D5100, and D90 all have 11 autofocus (AF) points with the center one being a more accurate cross-type. These AF systems may be more than sufficient for most users, and they can successfully track moving objects in the frame such as athletes, performers, or animals. However, if you specialize in sports, action, wildlife, or bird photography, you are going to want to consider the much more sophisticated, accurate, and customizable AF system of the D7000. The D7000 boasts a significantly improved AF system of 39 AF points with 9 of them being cross type. The AF system of the D7000 allows for you to use these points in various ways including automatic AF point selection, single point AF, and dynamic area AF using your choice of 9 points, 21 points, all points, or all points with 3D-tracking. It is important to note that the D5100 and D3100 do not have an autofocusing motor built into the body so you have to be sure to purchase lenses with built-in AF motors. For example, the Nikon "Nifty 50" 50mm f/1.8 will not autofocus with these cameras. The D7000 and D90 have the built-in motor.
Body, Construction and Size/ Weight: The D5100 is just slightly larger and a tiny bit heavier than the D3100, both weighing just over one pound. Both have plastic bodies and more limited buttons and controls that the higher end models. Many users should find its size and weight great for using and carrying around, though some prefer the ergonomics of a larger body. The D90 and D7000 appear very similar at first glance, but the plastic body of the D90 has been upgraded to the partially magnesium alloy body (top and rear) of the D7000. This adds slightly to the weight: 1.5 lbs for the D90 vs. 1.7 lbs for the D7000. The D7000 also has weather sealing at the memory card and battery doors, which the D5100 and D3100 do not. The higher end D7000 and D90 include not only the 3" rear LCD screen but also a top LCD panel for viewing and changing your settings. This is essential for photographers who are constantly changing their settings to deal with various shooting situations. For most users, including even those using the camera daily or in travel situations, the non-magnesium construction of the D5100 should be far more than good enough, strong enough, and durable enough.
ISO: As mentioned in the Sensor and Image Quality section above, the high ISO performance of the D7000 is greatly improved over the D90. The tests at dxomark.com tell this story, along with the fact that the native ISO range of the D7000 is 100-6400 expandable up to 25,600. The D5100 shares these specifications, and should offer similar results. The D3100 has a native ISO range of 100-3200 expandable to 12800, and the range of the D90 is 200-3200. This means that with the D7000 and D5100 you can use higher ISO settings when required, such as in low light situations, and not have as much difficulty with digital noise, particularly in the shadow areas of images.
Controls: As with construction, the buttons and controls vary with these cameras. The D3100 and D5100 offer more limited, basic controls on the exterior of the camera. However you can use the rear LCD screen to quickly change many settings, or else go into the menus. The D7000 offers an extensive array of controls on the camera body, allowing one to quickly change an large number of settings as they work, including focus mode and focus area settings, shooting mode, and exposure mode. The controls of the D7000 are similar to the D90 with some changes including the addition of the shooting mode ring under the mode dial (to change from single shot to high speed continuous to self timer, etc.), and the live-view switch with movie record button inside it. The D7000 also offers 2 customizable user settings (U1, U2) on the mode dial, and you can assign functions of your choice to buttons such as the Fn Button.
Menus and Custom Settings: These allow for greater control over customizing how the camera functions. The D5100 has less Menu and Custom Settings options than the D90 and the highly customizable D7000, and more than the D3100 (which offers no custom settings). These settings enable you to customize the operation, function, and controls to work how you want them to, including things like exposure increments, Live View options, tweaking how the autofocus system operates, setting more precise white balance settings, and customizing which button does what. There are ebooks such as my Nikon D7000 Experience - The Photographer's Guide to Operation and Image Creation with the Nikon D7000 and Nikon D5100 Experience - The Still Photographer's Guide to Operation and Image Creation with the Nikon D5100 which walk you through all of the Menu settings and Custom Settings so that you can set up your camera to work best for how you photograph, and also begin to learn to master all the advanced features, settings, and controls of these powerful dSLR camera.
Wireless Flash: The D7000 includes the feature of advanced wireless lighting using the built in flash as a commander for off-camera Nikon Speedlights. However, the D5100 and D3100 do not have this capability. With the D7000, you can set up one or more Speedlights in remote mode, then trigger them wirelessly with the built in flash of the camera.
Viewfinder: The D5100 has a pentamirror viewfinder with approximately 95% coverage of the actual resulting image, the same as the D3100. The higher quality pentaprism viewfinder of the D90 gives 96% coverage of the actual resulting image, while the D7000 has an even larger, brighter pentaprism viewfinder with 100% coverage. Author's note: this review was written and posted on Amazon by dojoklo - please do not steal it, post it elsewhere, and claim it as your own writing. Thank you. While in-and-of-itself, a 95% viewfinder works just fine, when you compare it side-by-side with the large, clear view of the D7000, you can see and understand the advantages of a clearer view of your entire scene with a 100% view, pentaprism viewfinder.
Processor: The Nikon D5100 and D3100 use the fast Expeed 2 image processor just like the D7000. This allows for more video options including full 1080p HD at 24, 25, and 30 fps, overall faster processing of stills and video files, and the ability to maintain fast continuous speed shooting for numerous frames. The D90 has the older Nikon Expeed processor, which is also generally fast enough to handle its processing needs.
Continuous Shooting Speed: As you work your way up the Nikon dSLR line-up the cameras' continuous shooting speed and maximum shots at that rate increases. The D3100 shoots 3 frames per second (fps) in continuous shooting mode, the D5100 shoots 4 fps, and the D7000 shoots 6 fps for up to 100 shots. The D90 can shoot 4.5 fps up to 100 images. If you often capture action and really need the higher frame rate, such as for sports, action, or wildlife shooting, you are going to have to seriously consider the D7000 over the other cameras. Paired with its advanced autofocus system, this fast frame rate can sharply capture moving objects is all types of situations. A nice feature of the D7000 is that you can adjust the low speed continuous mode to shoot anywhere from 1 to 5 fps, using the custom settings.
Memory Card: The D5100, D3100, and D90 all use a single SD memory card. The D7000 accepts 2 SD cards, where the second card can be used in a variety of ways: overflow when the first card fills up, JPEG on one / RAW on the other, or mirrored backup of the first card. The second card can come in handy as well if one is shooting video files, and one card can be designated for stills and the other video.
Battery and Battery Grip: The D5100 and D3100 both use the EN-EL14 battery, and the D7000 uses the new, higher capacity EN-EL15 battery. The D7000 accepts the optional MB-D11 battery pack/ vertical grip which is constructed of magnesium alloy. The D90 uses the EN-EL3e battery and its optional battery pack/ vertical grip is the MB-D80. The D5100 and D3100 don't accept a battery grip. The battery grip is handy for providing the ability to use a second battery and thus prolonging shooting time, and also creates a larger camera body which some users find more comfortable, especially when shooting in portrait orientation.
Full HD video: The D5100 shoots 1080p and 720p video at 24, 25, and 30 fps. The D3100 shoots 1080p at 24 fps and 720p at 24, 25, and 30 fps. The D7000 also shoots 1080p at 24 fps only and 720p at 24, 25, and 30 fps, up to 20 minutes with full-time continuous autofocus. The D90 offers 720p video at 24 fps, with a 5 minute shooting time.
Ease of Operation: While beginners may find all the buttons, controls, and menus of any dSLR difficult and confusing at first, the menus and controls of the D5100 and D3100 are pretty basic and simple to learn for a dedicated user. The additional controls and menus of the D7000 and D90 are all quite intelligently designed and will become intuitive and straightforward for the more advanced user once they are learned and understood. Again, have a look at helpful guides such as my Nikon D7000 Experience - The Photographer's Guide to Operation and Image Creation with the Nikon D7000 and Nikon D5100 Experience - The Still Photographer's Guide to Operation and Image Creation with the Nikon D5100 to begin to learn to master all the advanced features, settings, and controls of these powerful dSLR cameras.
Hands on Experience: The camera body of the D5100 fits nicely in the hand, and is a little bit taller than the D3100, so the pinky doesn't fall off the grip quite as easily. The rubber gripping surface and rubber thumb spot work nicely, and help retain the grip on the body, even when holding it at your side or carrying it around without a strap, and the light weight of the camera also adds to this ease of portability. Those who prefer the ergonomics of a larger, more solid feeling camera body will have to look at the D90 or D7000. The Live View switch, placed on the top of the camera at the mode dial, feels and sounds a little plasticy, but works find, and the record button for movies is conveniently placed on the top of the camera, near the shutter button. The FN button, on the front near the lens mount, can be customized to adjust one of a number of settings quickly, like image quality, white balance, HDR, or +RAW (to take a RAW file in addition to a JPEG if set on just JPEG). I prefer to make it an ISO button. The side mounted rear LCD screen is a definite improvement over the bottom mounted, limited screen of the D5000, and rugged and durable. The view through the viewfinder is a bit tiny and cramped, as is typically the case in this level of dSLR. Changing settings is quick and easy with the "i" button and rear LCD screen. Overall, the body, feel, controls, and LCD screen of the D5100 make for a great image taking experience.