I also obtained a D3100 a few weeks ago. I have previously used A200, 1000D and D60.
Basically I don't like a few things. I wish future firmware releases will do something about that.
Autofocus: I have a high rate of our of focus photos with the kitlens AFS-18-55. I am using single point, single servo focus and the shots are mostly from still, non-moving objects. So this cannot be because of incorrect AF setting. I am forced to shot with higher F to decrease the problem.
Out of focus shots are much more than what I used to have with 1000D (with 18-55) and almost equal to A200 with 17-80 and 55-300.
Auto Mode not usable: The other day I was convincing my friend that D3100 is enough for his need and he does not need to pay much more to buy a 550D.
We visited a shop, inserted our own SD cards and put both cameras on Auto and took 30 shots. Later I found that D3100 has taken all the photos with Hi2 (ISO12800) while 550D has selected ISO400-ISO800 exactly in the same situation with an almost the same lens (both with 18-55, F3.5-5.6).
As a result shots from D3100 are terribly noisy. My friend immediately selects 550D. I myself never take photos on Auto-Mode but if I ever want to hand it to an inexperienced user to take photos the problem will hit me too.
Metering with open Flash incorrect: With closed flash in a specific situation, camera would auto select ISO3200. Opening the flash does not have any effect and camera still auto selects ISO3200. Ken Rockwell refers to the same firmware bug in his review.
Underexposure: My camera underexposure a lot of times. As a result I have a +0.3 exposure compensation setting most of the time.
Other than those mentioned above I love this camera. It provides much more than its price. I am very very happy about the ISO performance, more happy than my previous cameras about White balance and I like the VR in kitlens. It is 2 times more effective than my EFS-18-55 (which possibly had IS problems?). With VR enabled I can take 1/8s photos without problem while on my canon 1/16s was the best I could do.
Mac