I had purchased one of the Micro Four Thirds cameras because it was light and small, but I found the quality lacking when compared to APS-C cameras. Also, most of the available Micro Four Thirds lenses are way too expensive for the quality you get with them; they're not that great. (The exception is the Panasonic 20mm lens - that's a good lens. Perhaps still a bit expensive.)
Enter the Nikon D5100. When I saw this camera, I knew it would be exactly what I was looking for. It's small, lightweight, easy to use but still maintains superb image quality. It's really outstanding in low light. I don't know what kind of algorithms Nikon is using, but shooting at ISO 1600 is no problem. Even the shots at ISO 3200 look good and clean up nicely.
The great thing about Nikon D5100 is that it's versatile and flexible. You can shoot in auto, or you can use Aperture or Shutter priority for more control. If you're an advanced user, you'll be happy for the manual mode that allows you total control.
Nikon D5100 really is the best of both worlds. A camera that an advanced amateur can use and at the same time feel confident that he/she can hand over to an inexperienced spouse or friend and know that the camera won't be too much for them to handle.
It's really a nice fit between the D3100 (more features, same sensor as the D7000) and less expense and bulk of the D7000. Plus, I love the flip out LCD.