Ability to see that others are also answering a question when you're doing so as well.

There has been more than once where I've posted an answer and had the page reload, only to see that somebody else already answered the question, sometimes better than I did.

Some sites will notify you that other people are also answering the question, or at the least, notify you when another answer is posted.

This is nice because it means you can check out the other answer while you're writing your own and stop working on it if the other answer is better than yours.