I have the 550D and 600D, and recently acquired a 5D Mark III (though it really belongs to my wife for photos). I only shoot video and couldn't have justified getting the 5D3 on video alone. While the 5D3 has clearly superior video capabilities than the 600D, I still use my 600D (a lot). 600D has better ergonomics for me and, with Magic Lantern, better "features". I've even been able to cut the 5D3 with the 600D where even sometimes I can't tell which was used after the fact.
Improved video capabilities (note that I didn't say features) would interest me. Here's my wish list for the 650D/T4i:
1. No moire
2. Improved effective video resolution (not sensor MP) to be true 1920x1080.
3a. Clean HDMI out (won't happen, but a guy can dream), or
3b. A 4:2:2 codec (again, a guy can dream)
4. Improved low light/high ISO performance
Note that none of these is what I consider a "feature". To me, they are capabilities that result in better images, not just easier images. I'm not interested in video features. I have plenty of those with the 550D, 600D, and Magic Lantern (thanks Alex!). A video AF is next to useless for me, and I take a lot of "live" shots.
If the camera only offers any of 1-3, I am very likely to get it (actually, one 3 option alone would sell it for me). Eliminated moire isn't a must-have for me, as long as they leave the 3x zoom in it (which eliminates moire, albeit at crop, but not resolution, cost). If it has any 2, I almost surely will get it. I'm looking forward to the actual announcement.
By the way, I just don't understand why someone who is getting a DSLR for video use would get a 550D rather than a 600D/T3i. The flip screen and the 3x zoom that eliminates moire are well worth the premium. For stills, I don't see why anyone would get the 600D over the 550D. I think the fact that the 550D outsells the 600D says why people buy these cams.