I've been nothing but happy with my iPad 4 over the last two years. The A6X chip with it's quad-core graphics is still a beast and it even beats the A7 chip on some memory graphics benchmarks. It's running iOS 8 great and outside of the different case design, the iPad Air can't do anything the iPad 4 can't do. Even though, the A7 was a stronger chip, without Metal, there wasn't much point since the A6X could run anything without breaking a sweat.
However, I'm now thinking it could be time to upgrade. And the biggest thing I'm excited about is the possibility of a lower glare screen. I remember when I upgraded to the iPhone 5 from the 4S and to my Retina Macbook Pro from a Macbook Air. The lower glare was a huge bonus and it was something that you could appreciate on a daily basis. The iPad Air suffers as much as the older iPads from mirroring light which can make it difficult to read text. That was my biggest disappointment when it was announced. Just this morning I was griping in church to myself how, as I used my ipad as my bible, it was reflecting the very bright overhead lights so I had to adjust the angle to read the text.
Also, now that I have my iPhone 6, I'm really starting to appreciate the TouchID. It is so nice to sign into Amazon etc without having to type my password. I'd kinda like that on my iPad.
And I also have a sneaking suspicion that multi-windowed apps will be an Air 2 only option that requires an A8X + 2GB of memory. I don't think Apple will allow such a memory intensive option to run on older ipads. I kinda like to listen to Youtube video's while I surf the web on my ipad and it would great to be able to do this on one device as opposed to running down my iPhone battery to do it.