Yes, lots of them!
* All icons on the home pages gone, so you'll have to re-organise everything, again.
* The UI is now the same one as on a phone, with a non-removable notification bar on top. Nice if you use a phone in portrait mode, but not if you use a tablet in landscape mode. (The bar on top makes the screen narrower, while the bar at the bottom is just lots of empty space.)
* To see the wifi, GPS, ect settings you now have to swipe down from the top, instead of the previous single tap in the right bottom corner. And if you don't swipe far enough down, the pop-up close again.
* I'm fairly new to Android and tablets, but I really started to like my Galaxy Tab2 7. But this new UI is just ridiculous, makes you wonder who they hire as HCI experts at Google...
To be clear: I didn't get any warning about the importance of this update. It got pushed through exactly like an update a few weeks ago, but that included some minor tweaks - nothing like this one. (I would compare this to Microsoft automatically pushing an upgrade from Windows 7 to 8 through Windows Update, without any serious warning...)
I have been using Android for 2 months only, but I think I'll be looking for Win8 tablets in the future.