Video Sharing Sites: Quality Comparison

YouTube
It doesn't really seem to matter what you feed it... YT's compression system will pretty much mangle anything. Expect heavy pixelation with any fast-moving clip.
According to their documentation, DivX is their preferred origin format, but it doesn't seem to help.
They don't currently accept Flash video, despite that being their native display format.
Pretty slow, encoding-wise.
OVERALL: They've got the traffic, but that's about it. Video quality is middling, with little reward in store for those who try to tweak.

MySpace
The quality you get out of their encoding process depends wholly upon your source clip. Give 'em a hi-res DivX file, and you get crap. Give 'em a good looking On2-compressed FLV, and you end up with something that doesn't look half-bad. Go figure.
Much quicker than YouTube at encoding new video.
They stick an obnoxious MySpace logo in the lower right corner of any uploaded clips... bear this in mind if you're running subtitles or something similar across the bottom of the screen.
OVERALL: Not bad at all, once you figure out that FLV is the secret key to quality video. The logo overlay sucks, but it's worth it.

Yahoo! Video
Uploaded an iPod-ready MP4, and the result was pretty awful.
Uploaded a hi-res MP2, and the result was pretty awful.
Yahoo! absolutely butchers audio. Music ends up sounding like an old AM transistor radio at the bottom of a well.
Decent encoding speed.
Bigger-than-average video window is nice, but just highlights the low quality of their transcoding.
OVERALL: Possibly the worst performer of all the video sharing services. Made even worse by the fact that it's Yahoo!, which should be able to afford better tech than anyone but Google.

Motionbox
Uploaded an iPod-ready MP4, and it took many hours for them to finish their transcoding. Once finished, the video quality was pretty rough, and the audio just barely acceptable.
Their "deep tag" system works like chapter-stops on a DVD... very handy.
They provide an innovative visual "scrubber", allowing you to scroll to a specific part of a clip without having to guess about timing.

Blip.tv
Finally, someone gets it right! Uploaded the FLV version, and it was immediately available. (Blip continued to do a transcode somewhere in the background.)
Their player scrunches the video down a bit, making small text harder to see than you might otherwise like.
First hiccup... the video stutters a lot. Like, every few seconds. Completely ruins an otherwise sublime experience. I'll try uploading an MP2 and see if that makes a difference.

Bolt
They accept FLV uploads... that's a good sign.
Offered the option of uploading a JPG "cover" for the video, effectively side-stepping the common practice of video services grabbing random (and often useless) frames and using them to represent the video during browsing.
No transcoding for uploaded FLVs, preserving the full video and audio resolution of the original. Looks and sounds damned good. Minor stutters here and there, but nothing compared to Blip.tv.
Player is sized appropriately, and no watermark.
Three words: kick ass service.

Google Video
No FLV upload... bad Google, bad!
Pretty much the only service out there that will allow you to upload files larger than 100MB. Granted, you're forced to use their upload tool in such situations, but that's better than nothing.
Uploaded a hi-res MPEG2, and Google transcoded it with remarkable speed. (Relatively speaking.) The resulting video was solid, though hardly spectacular, and kinda soft. Let's call it "better than average".
Google's tagging system is cumbersome compared to the competition, and they're only now adding features like comments.
On the positive side, they're one of the few services with a "download to iPod" option. No need for iTube here.