The 5D Mark II is groundbreaking in a bunch of ways... a 21MP full-frame sensor than can shoot at ISOs up to 25600 (!!!) for $2700? That's amazing.
But what I'm blown away by is 1) it shoots 1080p video, and 2) how good the video is, especially in low light. THIS is game changing. Go watch the movie Vincent Laforet made with a prototype. Keep in mind this is unedited footage and it was shot at ISOs 1600 and higher (the only change is being re-compressed at lower resolution).
The movie (on Canon's site): https://www.usa.canon.com/dlc/controller?act=GetArticleAct&articleID=2086
Now go visit Vincent's blog and read up on the whole thing... it's pretty incredible. You can also download original raw footage and stills.