I want to thread comments: when a comment quotes another comment it's probably a reply, so attach them together to make the conversation clearer. This means figuring out which comments quote which other comments. I tried a brute force way:
Update 2012-07-21: Several commenters suggested a better algorithm: to find all quotes of length N, build a dictionary from all sequences of N words to a list of comments in which they appeared. This is O(n*m) and if you run it (slightly_smarter_threading.html) you can see it's much faster. I tested it in IE8, and it loaded quickly instead of freezing the browser. I thought briefly about doing something like this earlier, but wrote it off as using insane amounts of memory. After more people suggested it, I realized it only uses N times as much memory as just storing the comments.
 If you're curious you can try, but fair warning: brute force threading.