If you're going to implement video do it as well as YouTube does it or don't do it at all.
When you go to a site you usually run into usability problems pretty quickly. They're not hidden. They're not complicated. They're not baffling. They were in the design or crept into the design.
Designers love subtle cues because subtlety is one of the traits of sophisticated design. But Web users aregenerally in such a hurry that they routinely miss subtle cues.
I don't think there are as many usability issues as there are tactical or strategic decisions related to whether incorporating social networking into your site is going to help or hurt.
The general reactions were that the video was either not going to load or be painfully slow to load or would require a plug-in users didn't have. YouTube changed that because it just works.