For me, the most painful is that this time they managed to screw up the damn keyboard while bringing absolutely nothing new. I can't even use hangout or chat on my iPad Air , i have to wait 3 seconds for my words to appear.
That's just wrong. There's no excuse for that. We're not talking about fancy animation or new features that we think aren't a great idea. Just a basic regression on one of the most fundamental things you can do with this device (the other being displaying things).
Another thing is that they usually fix slowdowns and stability with the following release soon after. Not this time, so my guess is that it'll be a "change your device" kind of upgrade.
