Well, speaking of iMessage, my 2 biggest annoyances right now are:
1. iMessages sometimes display out of order. I'll send one message, then another, and the second one will display first. Happens a lot when I send images as well. I recently switched to a new phone, fresh install, latest OS, didn't restore my old stuff, and it still does it.
2. When sending pictures from the camera roll, the first one will go through fine, then the second one will freeze up iMessage. Eventually it unlocks, if you wait long enough, and then half the time it has trouble sending it.
2a. Sometimes when I click on the camera icon from within iMessage itself & select a picture, it either lags, locks up, or doesn't actually send the picture, and I have to go & click on it again. This happens a lot less than #2 tho. #2 happens pretty consistently.