One thing I've noticed, in addition to the differing quality of the encodes, is how each site handles URL's.
For my most recent Vlog Soup post featuring Bre Pettis of Make Magazine, I visited each site to see the results.
YouTube - Complete text with clickable links. Nice quality letter-boxed video too.
Yahoo! - Complete text and URL's are not clickable. Nice quality letter-boxed video.
MySpace - Complete text and URL's are not clickable.
MetaCafe - Complete text and URL's are not clickable.
Revver
Dailymotion - Complete text with clickable links
Brightcove - Doesn't include the complete text and URL's are not clcikable.
I manually uploaded this video to blip.tv. When you use TubeMogul the URL's don't become clickable automatically, but you'll notice that blip.tv does support wide screen.
I'll have to visit the sites to see if adding HTML will make the URL's clickable, but it would be nice if they were automatically made clickable like on YouTube and DailyMotion.
Conclusion
For my first test, YouTube and DailyMotion win for best text formatting, maintaining all the text of the post, and for making the links clickable.
Blip.tv has the best support for wide screen, keeping the original size, plus they give you access to the original QuickTime file for viewing online and for RSS 2.0 with enclosures downloads. YouTube adds letter boxing to provide the second best video experience.