I wish this had a simple answer, unfortunately it doesn't.
Firstly it all depends on what you start with. If you start with blu-ray quality HD like 24Kbps 1080 video that is a good start.
Did you produce your video to the same format and quality as the raw footage?
Lastly, a little story. Last year I found some youtube footage which just blew me away with the quality even playing it back through youtube at 320 let along 1080.
I am talking stunning quality. I actually traced down the name of the camera used and found out it was a $42,000 professional video camera. So you see it also depends on what you start with as to what you end up with.
So to sum a few things up, bit rate is usually more important that video size eg 720 or 1080.
All of this doesn't even touch on codecs or formats used to produce your final video.
Lastly did you produce your video then manually uploaded to youtube. I say this because the default settings using PowerDirector to automatically upload to youtube is not the best quality.
I am getting reasonable youtube footage with my GoPro HD shooting at 920 at 12Kbps and stretching to 16:9 aspect ratio then producing to H.264 15-20Kbps and manually uploading to youtube.
Cheers
Robert2 S
My youtube channel====> http://www.youtube.com/user/relate2?feature=mhsn