An unpolished quickie : I'm typing over a dinner plate.
Twitter is brilliant, amazing, game-changing, revolutionary. It's winning features have been discussed to death, I don't need to analyse that here. But it has a couple of really glaring failures.
For one, it's impossibly inconvenient to reconstruct a conversation between two people if you don't follow both of them. So your friend is having a conversation with someone you don't know, and it's on your favourite topic, and you can't really meaningfully participate, nor even easily historically reconstruct the conversation. It sucks.
For two, it's single-channel only. You can't send a tweet to a subset of your followers. If you want to tweet about personal stuff to your daytime friends, and tweet about programming stuff to your geek friends, and about party stuff to your pervert friends, you have to set up separate twitter accounts for each group, and have everyone follow the appropriate sub-selection. So hard to manage that nobody does it with a granularity of greater than two. Most people don't even do it at all. So everyone's signal-to-noise is ramped way up, because the topic you follow anyone for is diluted by all the other shit going on in their life, most of which you usually couldn't give two fucks about.
Fixing both of these would be REALLY frakking awesome. Would really expose all the nebulous potential that Twitter users can see right there just beyond reach. Twitter itself won't do it - they are too afraid of souring the magic sauce that has given them such amazing penetration. So expect a Twitter-killer, sometime this year. Incorporating some sort of equivalent of chat rooms or private channels.
Funny thing is, once you do that, what you're left with starts to look an awful lot like IRC. Like the man on BSG says "All of this has happened before, and will happen again, and again, and again."