3rd party keyboards suck on iphone. There are small things I dislike very much about iphones keyboard and Googles keyboard fills those holes, but some times the google keyboard comes up some times the iphone, it's a crap shoot.
I hate how it sorts the "desktop" of the phone, if I want to swap an app on my home screen I have to spend way to long rearranging the apps because it always moves everything down one spot.
Not a fan of the notifications, although it has gotten a lot better.
those are the two things that annoy me the most, but there are other small little things that may seem nit picky to some but when you're on a OS that allows you to change something that seems simple and then go to one that doesn't it's VERY annoying.