My biggest gripe is the occasional time-out errors when you're in the middle of writing a post.
That and the inconsistencies around entering text and uploading attachments. It feels like that button bar above the text entry window was a cobbled-together after thought, designed by totally different teams and then they couldn't even agree whether to group them on the left or the right, so they just put half and half. Which results in the ability to add inline attachments separately from stuff that sits at the bottom of the post, but sometimes that doesn't work and sometimes it works for you but others don't have permissions. Bleh, it's just whack. And the whole profile thing seems over complicated as well.
But I do love that people's personality comes through here, both with their avatar and username but also their post count, reputation and groups. It encourages long form conversation (like our debates) and that seems to go hand in hand with more thoughtful conversations, which is distinct from Twitter and something I value highly.
Edit: Sorry, that wasn't really one thing, was it?