Deepak Thomas and Nisan Gabbay post an excellent and detailed "YouTube Case Study" that analyzes the reasons for the success of the startup.
To summarize, YouTube eliminated the hassles with sharing and watching videos by using the just-released Adobe Flash support for video, then drove rapid adoption through offering embedded widgets and distribution of copyrighted content.
Deepak and Nisan also suggest at the end that there might be an opportunity for video startups that offer higher resolution video than YouTube.
I suspect a bigger opportunity lies with helping people find and discover interesting videos on demand, effectively helping people create their own TV stations customized to their own interests. Lack of item authority and poor recommendation quality make this very hard to do on YouTube right now.
See also my previous posts, "YouTube and ease of use", "YouTube cries out for item authority", and "YouTube is not Googly".