I want to think that it helps to have pretty, descriptive URLs, but things like YouTube make it seem like it really doesn't matter what the URL string is – as long as users can copy and paste it and link to it, anything beyond that is a waste. On one hand, I guess it can be nice to see what the content of a page is before you follow the link (title slugs on blogs) and it would help you avoid getting RickRolled, but on the other, it could teach you to get used to trusting that the URL slug actually matches what's on the page. There could be anything there, really. It's up to whoever has control over it.
Anyway, I think YouTube's random string/hash URLs worked out fine for them. They're short, easy to copy and paste, and they make you think twice before clicking :]