Hi Nadye!
My biggest complaint is Entourage doesn't feel "Mac-like." For example many of the controls are not Cocoa so they look and feel a little "weird" and sometimes do not operate the way I would expect. One example would be when composing a new message, none of those controls are Cocoa. So I can't use Cmd+Ctrl+D to quickly look up a word with the OS X dictionary, it doesn't follow my system-wide preferences with regard to spell-check, copy+paste alters formatting sometimes, things like that. While certainly all the functionality is there, it is strange to have to use "Microsoft Dictionary" and "Microsoft Spellcheck" with different shortcuts and methods when I'm used to doing it everywhere else with built-in OS X functionality, inherited from Cocoa.
Another thing that bums me out a lot is the automatic text wrapping that Entourage applies to plain-text messages that I compose. It would be nice as an option but I don't like that there is no choice but to send rich-text e-mail if you don't want your text wrapped.