It's easy to spread yourself too thin by trying to maintain an online presence on a lot of similar community sites for hosting content. They all have different designs and by virtue of their architecture or just the vibe the creators and the initial group of users create they seem to be good for different things. But I'm trying out vimeo because of their focus on user created content- there's a lot of artists in there, and it sort of has a flickr feel to it- and the video quality is pretty good, the UI is nice that it disappears when not in use (though it obscures video when in use). I'm just hosting files identical to ones I have on myspace, google video, and youtube, but whatever site feels the most productive I'll probably eventually use to the exclusion of the rest. When someone starts a free 640x480 or HD hosting site I'll probably re-evaluate...
Youtube is the most popular, but video quality is garbage and most of the content is garbage. I imagine the user base is on the more youthful side.
Google video is better quality, though has no real community feel- it's just a faceless generic place to host video. Some day I might be interested in trying to make money off of uploaded video that I own entirely but for now that idea is mostly incompatible with my approach to this sort of thing (a tip jar feature with proceeds donated to a charity of my choosing would be nice though).
Myspace is not primarily a video sharing site, but they have a decent video uploading feature. If you have a myspace page and make video you might as well use their hosting instead of embedding someone else's. It can feel a lot more personal even if your page is not your personal page because of the emphasis on friending.
There's a ton of other hosting sites I won't bother with, mostly since their user base is marginal or they are just a dumping ground for copyrighted material, racier stuff if their content policy is more liberal than youtube or google's. Others are completely based around making money for uploaded content, pay per view basically: get thousands of views and make $10 or so. That sort of goal based emphasis probably marginalizes a lot of otherwise interesting content, encourages copy-cats and therefore homogeneity. So does having a view-counter or ratings (read slashdot for a few months to see how their 'karma' promotes pointless regurgitation...), but money is a force-multiplier.