The lack of design supervision for the back-end.

We have plenty of coders, plenty of theme designers, but aside from the awesome Happy Cog-designed dashboard a while back we don't see much of WP's backend UI aspect being discussed. I feel that there are plenty of areas that can be improved there, down to the smallest things (ever tried Quick Editing a post? You click the save button and whoa, the loading icon shows up to the far left where 1) it could easily be missed and 2) it forces your eyes to move unnaturally from right to left).

Probably not the weakest link, but still.