Better support from Manufacturer and Accessory makersPepto said:Why?
Higher bitrate: Canon ~45mbps vs D3100 ~19mbps
Higher res LCD: T2i 1040k vs D3100 230k pixels
HDMI live view
1080p/25, 720p/50/60
Mic input vs no Mic input (although you'll probably want to go external at some point)
Manual control for video not confirmed yet.
Continuous AF isnt a huge selling point if you're making films. For just videoing little Johnny at the park, it will be helpful