- Dual Control Wheels
- Top LCD
- Bracketing
- In body focus motor (for old AF lenses)
- Flash Commander Mode (not on D3100 I think)
- Pentaprism
But both are good cameras.
Just saw this list and I have to comment.
While the D3100 is "newer" tech and whatnot, the D90 is still a remarkable camera with more powerful features, even though it is an older model. I owned a D90 for over a year before it got stolen and that camera was amazing.
The 2 biggest differences that will matter to you?
1. The in-body focus motor. Since the D90 has a motor in the body, you can use both G and D lenses on the D90. G-class lenses are newer lenses and have focusing motors built into the lens. A D-class lens does not have a focusing motor built into the lens, so if you try to use a D lens on the 3100, you will have to manually focus it. No autofocus. Why does this matter to you? You can buy some amazing D lenses for a lot cheaper than G lenses.
2. The Top LCD. It uses very little battery power and displays a lot of useful information without you needing to waste battery on the LCD at the rear to see this info. Why does this matter to you? Being aware of your settings when taking photos will help to improve your photography, and saving battery is always nice. I sometimes left my D90 on for weeks in the bag and still had battery left over to shoot with.
Still, I see you did get the D3100, hope you enjoy it.
More often than not the photographer matters more than the equipment.
