Twitter is not about features, is about lacking them *and* the fact that everybody else is (or seems to be) there. Downtime is a non-issue for anyone but the hardcore twitters, but even those are not willing to change.

The only way I see something gaining critical mass up to a point of menacing twitter would be if it was something bigger, with a good-enough Twitter functionality. Facebook is the obvious candidate right now, but could be Google Wave. Or maybe microblogging losing its lust. Other than that, Twitter is the way to be – sorry, dear.