Everyone who asks me about Twitter says the same thing: "I don't really care what people are having for lunch." And it's true that a lot of folks are using Twitter to say what they're doing at the most inane level possible. But just as the people on a single bus are all going different places, tweeters are using Twitter in very different ways: to ask questions, to inform others about developments, to give their opinions on major issues, to report humorous occurrences, and, most importantly, to establish two-way communication. Twitter is the only wide-open social networking platform that approximates public conversation, because you don't have to be "approved" to listen in. Unless someone has protected their updates (and few people do), anyone can follow them.
Of course, this leaves Twitter wide open to marketers and porn site spammers. You can block tweeters on an individual basis, but it is annoying to have to keep clicking the block link when you get the email and see who is following you. The marketers I don't care about. I don't generally follow them back, but I don't bother to block them; I suspect most of them un-follow me once that's clear. But this is a minor annoyance; Twitter support usually finds a way to block spammers after a day or two.
The Iranian political situation highlighted Twitter's strengths in getting important info out quickly. The recent Worldcon in Canada showed Twitter's addictive nature; countless American attendees, cut off from cell phone access to the web, whined most about not being able to tweet or to read their Twitter pages. The power of Twitter lies partly in the public stream— all unprotected tweets, streaming past by the thousands— partly in the trending topics— the automated Google-like filter that shows what topics are being tweeted most— and finally in the ability to customize your list of whose tweets you see.
When people ask me what Twitter is, I say, "It depends on whom you follow." You can make whatever you want of Twitter by looking for interesting people to follow. And if you want to, you can throw your own voice into the mix and report things you think people should know.
And finally, Twitter is fun!