I look at it this way, and remember this is without actually having used one yet: I don't believe there will be a whole lot to complain about when using the iPad Air. For example, weight will no longer be an issue. The screen is obviously retina, so it's going to be as sharp as you can get. Everyone who actually got their hands on one and reviewed it praised its zippy speed and how well it functions with iOS 7.
This is compared to previous models where when you picked them up and used them, you immediately picked up on faults before there was any "newer" technology to compare them to. The iPad 3, for example, was heavyyyy. If you planned to use it as a reader, I think you quickly changed your mind unless you wanted to use two hands. For one hand, it was simply too much on the wrist. Also, the iPad 3 was painfully laggy and slow (even more so once iOS 7 dropped). These faults were a major reason why the 4 was pushed out so soon after.
That said, when you look at the Air, sure you can say that more storage would be great. Touch ID would be great. And there will be other little things. But as far as the major checklist goes, the Air seems to have nailed everything based on early reviews and the keynote. In other words, it will more than suit your needs for a while to come, assuming it will take to iOS 8 much better than some of the previous models took to iOS 7.
And obviously Apple will come out with new technology that will eventually leave the Air in the shadow. But as far as necessities go (weight, speed, screen, etc), the Air seems to have nailed them all.
That was much longer than I intended, but you ask an important question when it comes to investments for the future like an iPad and I feel like Apple did a great job of setting up the Air for great longevity.
Edit: Same argument can be applied to the mini as well. Just realized you said iPads and not just the Air.