We've entered a period where technological improvements have slowed a bit. I'm still using the dSLRs I bought in 2004 and 2006. My laptop is 3 years old and does everything I need. I expect my iMac to go 4 years or so. I'm not sure why anyone thinks this is a bad thing.
I upgrade my iPhone and iPad every year, handing the old one down to my wife and her old device down to my 5 year old (plus an otterbox

). So..I've got a lot of iOS devices in action at my house, and honestly, there's not THAT much difference even across 3 generations (3GS vs 4S). The new stuff is a bit better all around, but they do mostly the same things.
Any improvement after the initial device is going to be incremental. That's engineering. Some more sensors, a slightly faster processor, some more RAM, etc. Apple is working with pretty obvious constraints (dimensions, weight, power consumption, cost) and no fantastic new technology has emerged and become usable in the last couple years. There's only so much they can do. And there's only so much that they need to do to stay on top.
Part of what makes iOS great is that the hardware remains constant and stable. A developer knows they've got tens of millions of almost identical devices to sell to, so they're happy to invest tons of time writing great code that's highly optimized for (at most) 8 devices but really only about 4 all running an identical OS. Your iPad is just a screen and a couple buttons. It's a box with lots of sensors and radios and it doesn't *do* anything. It's a dumb brick without apps and those apps continue to to improve at an astronomical rate.
Compare that to Android. Sure, there are some Android phones/tablets that have faster hardware and more memory. Why? BECAUSE THEY NEED IT. They need to compete on specs, and since the OS isn't as tightly constructed a lot of the RAM and CPU cycles are used just to generate heat. The result is rotten battery life, inconsistent software and general linux-like levels of ******** across the board (this is being typed on an Ubuntu box, for the record).
Apple knows what it's doing.