The title pretty much covers it. Twitter is more like an IM network or email list service than like a typical database-backed webapp; a relational database is just the wrong platform for large-scale messaging.
I agree, of course. But before the twitter bashing starts, I wanna say, I don't think that they foresaw messaging being the primary use-case. After all, you already have IM and SMS and a gillion other messaging systems. Twitter was conceived as a blogging platform, but smaller. Messaging was how you would post to twitter, not what you would do with twitter.
Of course, that was wrong. (How annoying are the twitter users who actually "answer the question: What are you doing?") But that kind of insight only comes out once your product is exposed to real humans. It's obvious, but only in hindsight.
As it turns out, when you make LiveJournal really small, you get Jabber.
