It sounds to me like you never played Diablo 2. The basis of that game was repeating through the same content (albeit with randomly generated maps) in search of better and better gear. This mechanic hasn't fundamentally changed in Diablo 3.
Diablo 3 seems to have an entirely different curve where progress diminishes to near uselessness until you get a lucky loot drop and you can finally make headway again.
In Diablo 2 there was always a "sweet spot" where you could earn maximum experience and you never seemed to be so woefully under-equipped. A surprising drop would mean a big boost in performance for the next few levels, always a treat, but if you didn't get one you could still do okay.
In Diablo 3 the theoretical sweet spot is always way ahead of what your gear can handle and you're left struggling, out of your league, or coasting in a less difficult area with near zero experience awarded.
It's just not tuned right, unless of course the idea is to drive people to the Auction House.
My character isn't "mine" any more. The only real way to progress is to grind for loot now. Or pay someone for it. (something my friends and I never did in d2)
I know that my skill set changed pretty radically on subsequent difficulty levels, the same tactics didn't work terribly well.
In D2 I'll admit the skill tree had some severe flaws, but it allowed unique customization to an extent.