It would be nice to have code completion for @param and @throws tags in Javadoc comments. That is, when the cursor is 
behind a @param or @throws tag, Ctrl+Space would pop up a window listing all the parameters or thrown exceptions of the 
Java element following the comment, limited to those that match a prefix (if any) before the cursor position. For 
@throws, pressing Ctrl+Space a second time would further extend the list to all imported exception types, then all 
exception types. You get the idea.

Maybe some other Javadoc tags could provide code completion as well. Also completing the tags themselves (such as 
completing @p to @param) would be helpful.