I haven't used IRC in years, so it may have changed, but the big wins:

* Stateful server. When you enter a room, you immediately see the conversation that has been going on there. That means it's possible to have conversations that are long-spanning (more like email) because you don't necessarily have to see replies scrolling by

* Integration with Stack Overflow accounts and reputation. Bad behavior is almost completely eliminated thanks to the tie in to the reputation system (you need some rep on Stack Overflow to chat, for example)

* Super slick user interface. You have to see it to believe it. Try it for a while and see.

FWIW, the guy behind mibbit.com has a partial solution to the 'stateful server' problem for IRC. In some ways though it's not a problem: it keeps things more informal. I think the ideal thing might be to get a little bit of history when you join.

The reputation thing is both a positive and a negative. It's going to be a barrier to entry for some people - both good apples and bad. Hard to know the ratio...

The interface is nice, but I like my IRC client too.

Its focused on the already established community of SO users - and it exists in the same place people are already hanging out answering questions. Plus the chat logs are being stored and made available the same way all the commons Q&A data is - I just think it'll turn out to be more and more valuable as they improve on it.