At its wide end of 18mm (27mm equivalent), the D90's kit lens displays about as much barrel distortion as you'd expect, and (like many consumer lenses) asymmetrically squeezes the left side more than the right.
Although I was able to capture some sharp photos, this was my typical experience: pictures were just not quite sharp enough, at least at the default Standard Picture Control setting. (1/100, f5.6, ISO Lo 1, 157mm equivalent)
The D90 performs quite well, with excellent noise characteristics up to and including ISO 800. Details begin to degrade slightly--but not significantly--at ISO 1600, and ISO 3200 looks a bit soft but is usable.
Interestingly, you actually get better photo quality in some ways at ISO 400 than at lower sensitivities, probably because of the processing algorithms that kick in with the noise reduction (compared with the default sharpness of the Standard Picture Control setting).
The D90 has excellent continuous-shooting performance for its class. The 11-point 3D AF tracking works pretty well, and I found it very responsive for shooting dogs and kids. (1/100, f8, 31mm equivalent, continuous L, 11-pt 3D matrix AF; cropped and scaled down.)
The D90 did pretty well retaining detail on white-on-white exposures like Oreo's fur. However, much of that's due to a tendency to underexpose on bright shots (next slide for more details on that). (Next slide: retouched inset for better visibility; 1/50 sec, f9, ISO 200, 67mm equivalent, SB800 flash in AA mode, spot metered off the eyes.)
Although the D90 has a tendency to underexpose shots with large fields of white, you can easily compensate during shooting. The right is the original, spot metered off the eyes, which should have produced lighter whites; the left is retouched to show the exposure I wanted and expected. (See previous slide for shot settings.)
This was a particularly tough shot that highlights the differences between the D90 and the more expensive D300. The D90, like other entry-level dSLRs, tends to produce some color noise and have posterized areas in exposures like these. Similar shots I took with the D300 and other midrange cameras don't exhibit these sorts of artifacts. The upper inset is the original; the lower is brightened for easier onscreen viewing. (ISO Lo 1, 1/100, f5.6, 147mm equivalent, spot metering, sRGB)