Twitter feels a bit like a 24/7 cocktail party full of intelligent and interesting people, where you can chose who to listen to. I think they've also evolved (or perhaps resurrected) a cultural norm of openness where you can listen to and engage in conversations with anyone. I think that, along with the asynchrony and discoverability, is what makes it slightly magical.
That being said, it has problems: - It can definitely be a waste of time.
- There aren't good tools for filtering, prioritizing, and contextualizing the feed.
- There's an excessive density of 'professional' social-media types who seem to spend all day twitterring about nothing but Twitter.
- I don't understand the appeal of the IM-type clients at all. They look pretty, but I can't imagine anything worse than being constantly interrupted by windows popping up all over my screen with random, disconnected tweets.
Unlike a social network, I don't need to be friends with someone to see what they post. I can stop following someone whenever and follow them again later. In short, it has the useful content of a social network without the spammy apps, useless profile information and dubious "friendships". It's a better version of irc.