Sony Vegas 10 Video made from smaller quicktime mov videos look crap on youtube
I would try uploading the video to vimeo as well. Or, if you have your own website, uplaod it to there and create a simple page with an embedded player. 

My experience is that YouTube sometimes adds a LOT of artifacts when it recompresses things. I've often experienced the problem you're having when I work with YouTube, but rarely with vimeo.  

If Vimeo looks closer to your source material and self-hosting looks very close to it, I'd bet that it's YouTube's codec more than anything. 

Good luck!