Quite a few things so far. It's not just because it's a different GUI from what I'm used to either.
Too flat, ugly. The screen is now full of nice looking 3D icons from the developers mixed with the Fisher-Price look Apple has given their icons. It looks like something for a kid's toy.
Hate the white type. Making it bold helped but I had to change my Wallpaper to something dark to be able to read everything.
Lack of a noticeable division line between items, especially in Calendar. The wasted space at the top when in portrait, and especially landscape view, a lot of wasted white space, it lists the days of the week, cutting off a lot of viewable screen space. Maybe I'm missing something but where is weekly view now showing the whole week rather than having to scroll sideways, like it is here
In Safari I like how it now goes full screen when you start to scroll down, but you used to be able to pull down and get the address bar, then touch the bottom of the address bar and it would scroll the top, not any more, you have to manually scroll all the way.
The home screen is still just a bunch of icons instead of useful data. The Android OS shows things like weather, stocks, news feeds along with app icons if you wish.
My overall opinion so far, meh. Flat and fugly Fisher-Price look.