The iPad 4 is still a great tablet and should be on form once iOS 8 is "optimised". Like iOS 7 the iPhone side was far more advanced than the iPad on initial release and took a fair few revisions to get the iPad working without lag/stutter (remember the 5 finger gesture stutter, yuck!).
Biggest issue for me is the performance of Safari. Why change it from iOS 7?
Ok, not too many problems but I hate the sluggish scrolling/transitions in the bookmarks menu, in the share menu 2 airdrop icons and text try to form on top of each other, 5 finger gesture sometimes stops working (swipe up of the control centre cures it temporarily) plus other user reported problems.
An option to stop the address and favourites bar from disappearing would be good too.
Battery life for me seems to have remained good with usage time of 14 hrs 7mins on continuous 3G cellular usage.