Just the changes to some of the fonts (such as how names look, that seems different to me), how buttons are presented (like Reply, Quote, Like, which seem to be just links now, rather than buttons), spacing and presentation of the top for alerts and search. It just seems less pleasing to (my) eye.
The dropdown used to be wider horizontally, showing all of the available forums. You didn't have to scroll every time through a vertical list to find where you want to go. I liked the wider version.
Functionality seems OK overall, but as with most upgrades of any software, it seems like a step...