Fuck the haters haha. The D3100 is totally fine for most video.
The whole not having a mic input? That's absolute horseshit, as I NEVER use the mic input on my cameras. I use a field recorder for when I need audio, and edit that shit in post. Quality is way better than the crappy AGC you get from the camera, and it costs no more than buying the most basic mic for your camera. Buy a tascam DR40 for audio, it's the cheapest way to get a decent field recorder with XLR inputs - aka you can use professional level microphones on it. Otherwise, the on-board mic's are fantastic.
The biggest issue with the d3100 is the framerates. You don't have 720/60p, and you don't even have 1080/30p... just 1080/24, and 720@24/25/30.
If you're not concerned with slowing your shots down for slowmo stuff, then it's not a problem, but if this is something you'd like? then it's not the way to go.
and anyone who says a Nikon can't take great video is just a shitty filmmaker. Nikons take fantastic video if you know what the shit you're doing.
Have a nice day, Bluename.