Ability to use keyboard shortcuts. I miss ctrl-K to add a link.

The little + to expand images.

Reddit Enhancement Suite has a field to add a note about a user. That's useful to tag crazies and people who run around accusing everyone of being triggered.

IRC chat? Why? Anyone can set up his own chat. All that does is make the people in the chat really insular. They develop their own slang and way of viewing the site and start crapping on anyone who doesn't know (which obviously they don't because nobody but them is in chat). And then they start getting into slapfights with each other, which spills over to the comments. Moderator wars, and just no. Set up your own.