A way to edit comments. I regularly have typos that I would like to go back and fix. Spell check, unfortunately, doesn't check for HTML typos, so I look like more of an idiot that usual in comments.
A search engine built into LJ, so I can do regex searchs on a users posts. You would have a user field, a couple date range fields, and a search field, with a check box if it's regex searches. There are a number of old posts of mine I would like to revisit, but I don't want to have to physically read through the million posts I have made to find the one or two I want.
An ability to go through all my old comments/posts in other communities and journals. A number of times I have written something witty as a comment to someone, and I wish I could go find those posts, but often I don't even remember who I was commenting to.
The ability to have LJ randomly select a user picture. I never think about using any picture other than my current default one. I wish instead of default, we could select random.
Intelligent cross posting. I think when you post, you should be able to have it post to several communities at once, and it would keep list of which ones it was posted to. Then when you look at the post, you would see a list of the communities it was posted to, but you would only see it once, even if you are a member of several of those communities. Why? Join a few photography communities and when you see the same post 20 times because the person is posting the same image to every photography community they are a member of, then it will make sense to you.