The D3100 has a very good noise profile; it matches (and perhaps bests) the current leader in the budget dSLR category, the Pentax K-x. It produces exceptionally clean JPEGs up through ISO 800 and very usable ones through ISO 3200. (Adobe Camera Raw doesn't yet support the D3100 and Nikon Capture NX 2 doesn't provide sufficiently granular noise reduction for me to do raw versus JPEG comparisons).
Normally, ISO 800 images of cat fur are useless because the fine, dark fur is highly susceptible to color noise. But these are very clean, and I'll probably be able to work with the exposure and sharpness once I have raw software support.
You can start to see some color noise in this shot, but it's still quite usable.
The D3100 has excellent color reproduction in its default SD mode.
For a relatively inexpensive lens, the 35mm f1.8 fares pretty well overall. There's a little bit of asymmetrical distortion on the right, however.
At its widest, 18mm, the 18-55mm lens tends to crush towards the right side of the photo. This was taken with the automatic distortion control off, the default.
As you'd expect, there's very little distortion at the wide end of the 55-300mm lens.
Although it has a 7-bladed aperture, the bokeh on the 35mm lens still strikes me as overly polygonal.