Testing uploading a 1080p video clip from GoPro to two famous video sites, YouTube and Vimeo.
Although YouTube supports both 720p and 1080p, but the HD video quality is unacceptable. It is due to YouTube's bitrate is limited to 3Mbps for 720p and 6Mbps for 1080p. If you upload a high bitrate video, YouTube will re-encode for you and the result is very bad. So if you are going to use YouTube for streaming HD video with high bitrate, you must know how to optimise the video before uploading to the site.
Vimeo's bitrate is limited to 10Mbps for 720p and 20Mbps for 1080p. For free user, you can only serve 720p video. So for my 1080p video, Vimeo will re-encode to 720p. But due to its higher maximum bitrate, or may be a better encoding server, the video quality is a lot better than the YouTube one.
Below is the testing video, IMHO the SD video on Vimeo is comparable to the HD video on YouTube. The embedded video from Vimeo is limited to SD, you must click to the website to watch the HD version.