I'm glad you asked!
To start with, the new iPad Air 2 has the latest and greatest SoC; the all new
triple core 64bit "A8X" (a higher clocked version of the A8 found in the iPhone 6 and 6 Plus), as opposed to the dual core A7 chip in the previous iPad Air. Only the new iPad Air 2 has this new A8X chip (the latest iPad Mini is still powered by the A7)
Included in the A8X SoC, is the all new 8 core customized GXA6850 GPU, coupled with Apple's new Metal graphics API, this new GPU completely mops the floor with just about every other tablet on the market. (Surface Pro 3 included!) This new GXA6850 GPU brings desktop/console class graphics and gaming to the iPad Air 2!
(Read more about this here
AnandTech | Apple A8X?s GPU - GXA6850, Even Better Than I Thought )
The new M8 motion co-processor and barometer.
2GB of RAM (coming from 1GB in the previous iPad Air)
128 bit DDR3 memory bus (coming from 64bit bus on previous)
2MB L2 cache (coming from 1MB in the previous)
4MB L3 cache
The new 8MP "iSight" camera (as on the iphone 6 and 6plus) coming from the 5MP shooter on the previous iPad Air.
New 802.11ac WiFi w/MIMO and enhanced LTE supporting 20 bands and "LTE advanced" capable of 150Mbps.
"Touch ID" (fingerprint sensor) security. This really works well, and trust me, with my recent experience with trying to buy a used iphone (for a secondary backup phone), and finding out that it was still tied to someone's account, found it impossible to get around. Apple does not play with security! I was quite impressed!
Thinner and lighter - at 6.1mm thin, the iPad Air 2 is the thinnest tablet in the world! And at .96lbs, I believe it also the lightest...
New upgraded Retina display with the same 9.7" 2048x1536 (264ppi) resolution, but now with more tightly bound layers and finally an anti reflective coating for better outdoor viewing.
And of course the amazing iOS 8 and all it's new features. iOS has always done a great job of staying "transparent", in other words, whichever app you might be using comes front and center, rather than the OS. (Windows Phone would be the opposite, where the OS is up front, and apps are second.)
So yes, quite a bit is "new" with the new iPad Air 2, while keeping the tried and true form factor that everyone loves. I do, anyways....