There's definitely a lot of unfounded hate for Diablo 3. It's certainly not Diablo 2, but I got my money's worth out of it.
The main problem with Diablo 3 is the auction house. Not even the RMAH - just the auction house in general. The main draw to an ARPG, and pretty much the entire endgame, is farming for better loot. In Diablo 2, you had to find all the good gear yourself, or make an effort to seek out other people to trade with. There was an entire rune-based economy that facilitated the trades, but you still had to go to the effort to set prices, find willing buyers/sellers, and complete the transaction.
With the Diablo 3 auction house, any piece of gear that you could possibly think of - and almost certainly better than any piece you'll ever find on your own - is available to buy with your gold. Some of these items cost next to nothing because the market is so flooded with gear. Why bother grinding for loot when you can get stuff that's so much better so easily? The auction house removed the one major motivator to keep playing like we all did in Diablo 2 - the hope for better loot on whatever you happened to kill next. Without that sense of excitement, there's really no point in playing long term.