-Better image quality and lower noise at high ISO with the Nikon D3100
-MUCH MUCH lighter, which will be nice when traveling
-Larger LCD screen
-11 focus points, including 3D focus which tracks the motion of the subject or camera.
-2nd Generation DSLR
-Quiet, quiet, quiet
-No bracketing option. Not a huge deal since I only bracket once a year
-Cannot use my old 50mm 1.8 lens. I knew this before purchasing the camera.
-As of 10/1/2010, Camera RAW for the Nikon D3100 is not supported in Photoshop or Lightroom. Need to shoot .jpg + RAW or use Nikon Capture NX2 for RAW files. That should be updated soon, so not a big deal.
-Can hear the lens focusing when recording video.
-No depth of field preview.
-Can't use wireless remote
I'm an amateur photographer and this camera meets my needs. I love the fact that it's small and light, with great quality images. If I were a serious photographer and had more money to spare, I would wait and check out the D7000, which appears to have phenomenal specs. I'm happy with my purchase and I highly recommend this camera.