I'm not sure if you have ever shot with Nikons? What I meant to say was switching manual settings in photo mode takes forever.
I currently have a Nikon D80 along with some video cameras and the D80 has a small settings screen on the top of the body of the camera. There are two dials, one each for aperture and shutter speed and a button for white balance. The D3100 has its manual settings inside of the menu screen on the main screen of the camera and with only one dial you can quickly change one setting but you have to hold a button and use the dial to change the other. This might not sound like much but when you are shooting skaters or something on the go its kind of a hassle.
On top of that (this all might just be me having never used the camera) I had trouble getting the manual settings screen to come up and in video mode I had to change my settings, exit video mode, and then reenter video mode to see the difference on live view (unlike the D7000 where it happens instantly in the live view).
I'm not a video expert but here in the link to the video test using auto and manual settings, no color correction, and 1080p 24fps. Also on my channel is a D5000 test and the videos for the D7000 and D90 tests will be up soon.
I'll try to embed but I can't usually get it to work, so here's a link:
http://vimeo.com/22176426
<object width="400" height="225"><param name="allowfullscreen" value="true" /><param name="allowscriptaccess" value="always" /><param name="movie" value="http://vimeo.com/moogaloop.swf?clip_id=22176426&server=vimeo.com&show_title=0&show_byline=0&show_portrait=0&color=00adef&fullscreen=1&autoplay=0&loop=0" /><embed src="http://vimeo.com/moogaloop.swf?clip_id=22176426&server=vimeo.com&show_title=0&show_byline=0&show_portrait=0&color=00adef&fullscreen=1&autoplay=0&loop=0" type="application/x-shockwave-flash" allowfullscreen="true" allowscriptaccess="always" width="400" height="225"></embed></object><p><a href="http://vimeo.com/22176426">The Sounds of Skateboarding. (Nikon D3100 Test)</a> from <a href="http://vimeo.com/gpearl">Gretchen Pearl</a> on <a href="http://vimeo.com">Vimeo</a>.</p>