I think the ability to have the theme selector available to guests is nessecarry*. Its a pain in the back side when your skinning IPB and you have to make that incomplete skin the default so that you want see what it looks like for guests (the login screen and warnings for example...)
As for the posting screen, i think it would be a good idea to have the smilies table and the test editior in one block to make things more compact. i cant describe it so i'll illustrate it instead.

Another idea is the abaility to switch from STE to RTE and vice versa in the posting screen (which ive mentioned before!)
Another thing which im sure ppl have mentioned before is to use AJAX with quick reply.
