My most hated bug is when the keyboard autocorrects, and you hit spacebar to accept the autocorrect, it doesn't put a space in after the word like iOS 6 does. This is happening on iPad, not sure if it's the same for iPhone. If you don't understand what I meant, here's an example:
type "cant wait" without the quotes in iOS 6, and you'll get "can't wait".
type "cant wait" without the quotes in iOS 7, and you'll get "can'twait".
EDIT: I just found out that this only happens on the keyboard in the app Tweetbot. Weird. Anyone else with Tweetbot that can confirm this? using it in iOS 7 on iPad 3.