It would be nice to be able to "follow" someone else's question without commenting on it, so you can see people's answers to a question you have, but that someone else has just asked, or just see answers to really interesting questions that you don't necessarily have an answer for.
I'd like a slap button as the report button for the n a u g h t y people... >:D And basically what everyone else is saying. The big one for me would be to be able to block someone who's bothering you, even if they're not following you. Also, to be able to unsubscibe from a comment conversation under an answer... :o
I'd like to be able to look up ALL the questions and answers on others' profiles, rather than just the recent ones. Some people have really interesting points of view, and I'd like the option to see more of what they had to say.
I'd like a slap-upside-the-head button for atrocious spelling, punctuation, and grammar from people whose first language is English. And a big, red flashing red indicator when a question is clearly racist, homophobic, or sexist. Sirens optional.