Well, **** like that is pretty much why the game is BoA now. It was simply far more effective to game the AH than to actually play the game.
Of current itemization, there are basically 2 glaring issues that jump out at me:
1) Mobs dropping anything feels way too rare.
Now, one of the promises of Loot 2.0 was that mobs wouldn't drop as much junk, but in turn, we'd get higher quality items. What we got was perhaps Blizz cranking drops back too far, while not making the loot itself good enough. Indeed, you are likely to find things while leveling a character to use as upgrades, but the problems arise at max level where more precise gear requirements directly affect what difficulty you can play in. And since some stuff now only drops in Torment difficulties, well, cross your fingers. It's pretty much so bad you see people instead resorting to running areas with high treasure chest populations, ignoring any and all monsters just to pop them and hope for the best. This resulted from a kneejerk reaction from Blizz to thin out chest spawns. Still doesn't change the fact that chests and select breakables seem a more likely source for loot than your average white monster.
2) Forgotten Souls are way too scarce.
This is perhaps an extension of #1, but it applies more so directly to legendary and set drops. 2 FS are needed for every legendary/set crafting attempt as well as 1 being needed for the rerolls of legendary/set equipment. Blizz implemented a "fail safe" where a player should be finding one legendary every 2 hours or so if they're actually killing things, but the problem is, a lot of legendaries you do find are going to be garbage. This puts pressure on crafting and rerolling to help players target gear they need if they also have the corresponding recipe to attempt it. However, just like finding a drop, most crafts will also be garbage.
Now, the trollish sorts will claim legendary drop rates are already too high and people shouldn't complain. Diablo is supposed be a grind, yada yada. FS, however, are not actual combat tools. Having 100 in your inventory will not let you clear T6. All they will allow is more endgame crafting attempts, which may or may not go in your favor. There are still dropped legendaries with superior and desirable bonuses, but I feel making the crafting process more casual friendly will up the game's longevity by virtue of allowing people to experiment with more builds and classes instead of sticking to one because that's all their time can manage to build.
If Blizz can tackle those, all the **** about BoA won't hold much weight as it shifts progress from luck and more to honest effort. Allowing trade, with or without the AH, just risks inviting all the cancerous activities back that outright led to people who rarely played the game itself influencing those who do. People love to bring up the D2 economy and how it made the game better, but when I look back, I just recall dupes ruling the scene and items most legit players would never find being the only things people actually wanted to trade for. When it came to things like Perfect Gems, the only time that flew was if you seriously had an account stocked with them like some kind of chinese gold farmer. No thanks.
Class balance, of course, is another story. Demon Hunters are my bag and I hate, hate, hate, hate, hate, how Cluster Arrow is pretty much the only reliable way to tackle later torments with any respect for time efficiency. When you have a skill that hits nearly 2x harder than anything else, that's just bound to happen. Some are okay with this just because they can still play the class. Some are okay with it because they like being an underdog and deliberately using other **** skills. Meanwhile, nobody really wants to invite a DH to anything because they either die in a hit with CA build or don't do enough damage with everything else. Alas.