A few more (I was in a hurry on the other post):
1) Time limit (10 min for your D3100) on the DSLR for video (some reported overheat). I just filmed a Karate promotion that lasted 1 hr 15 min. Can't do it with the D3100.
2) Ergonomics - much easier to grip and pan, especially when you have a fully articulated LCD screen. I believe your D3100 has a fixed LCD screen.
3) External mic input and audio output
4) EVF to use in sunlight. I believe your D3100's VF goes black in video mode.
I'm sure there are more, but that's all I can think of for now.