youtube dumbs down the videos when you upload them. Some accounts, the big guys, seem to get good quality, but most don't. I only record in 720p, and upload an MP4 720p file to youtube, but they never look as good as they do on my computer. The HERO 3s will do 1080p, but even the black isn't "great" quality HD, but it is pretty damn good.
Youtube always tells me to "adjust your quicktime setting for better results." I don't render the videos as a quicktime file though, i use the MP4 HD settings, which aren't that great to. I was going to try rendering a Coral pink 4th video as a quicktime file and see if it helped. I think Youtube video quality has gotten worse really.
There are tons of formats to render a video in and they all have advantages, my PS3 will play the MP4, and my computer seems to play them well so that's why i use it.