Things to fix on YC News
Also, replying really shouldn't expire at all, let alone within an hour or two of the post being opened.
Unicode in comments would be good.
Better markup options. Quoting especially should stand out. You can't add literal asterisks for the most part. Code should be less of a pain -- you can't start a comment with code because leading whitespace is eaten.
Better user history (not just the last X comments, but all of them, so we can find what we wrote again)
Notification of responses so we don't have to check our own threads constantly
Private messaging, instead of responding to someone and hoping they check their own threads and read it
The ability to vote stories down -- this can help separate out blog spam from useful new submissions that just haven't gotten attention.
Similarly, the ability to vote down direct replies. The current lack is probably to prevent mere gainsaying, but it doesn't work because there's currently a disparity where someone else can vote you down and then reply, while you can't vote down what they are saying. First move loses. In both of the above cases this doesn't preserve karma as you can always vote down their other, unrelated comments; it just stops you from expressing your opinion.
You should be able to kill your own posts at any time. Restricting editing is one thing so someone doesn't twist something around on you, but you should be able to delete anything you post just in case (maybe you messed up with an NDA or other confidential information; realized you made a mistake on something an hour later; other legitimate reasons will occur to you).
Reddit is a pretty good model for the basics like this.
Others?