Something that annoys me is that there is no default (!) way to check who's online without being OP. So my suggestion is: The ability to press TAB to bring up a GUI with list of players.
The ability to bring up the 10 most recent chat lines in the chat (you press T to open the chat and then you press ARROW UP to "scroll" through the 10 most recent chat - the first one being the most recent one). I think this would be good because with the current system you have to write the command again if you want to use it more than once in a row (yes, you can use CTRL+C and CTRL+V but it is too much of a hazzle IMHO).
The ability to scroll through CHAT (using PAGE UP and PAGE DOWN).
EDIT: It's being added next update!
Give me the link, please.