Twitter forces brevity and conciseness of communication which is often a beneficial attribute...and it's something which neither irc, nor email, or blogging do. RSS, which DOES shorten things, has a lot of fail when it comes to typical data sources (like blogs) which were not written with the intent of being short and so lose fidelity.
Twitter also can be used with built in sms on phones easily and quickly. Email can, too, but you have to select a distro ahead of time...which loses twitter's second communication value...reaching people you might not have thought were interested in the subject at hand through subscriptions. You can do this with irc in general channels, but most phones dont have built in irc clients and so doesn't and will never have the user base of a system that you can use in every SMS capable phone without additional application installs. You'll just never ever have the same kind of usebase in irc without a drastic, radical change in the market.
Finally, the irony of people biatching about how boring or useless twitter is have largely themselves to blame. If you know interesting people, they typically have interesting, useful things to say. If your friends are all doormats, well, theyre going to talk about what they had for breakfast today. And the weather. And etc.