In Diablo, the real problem was the loot itself. They were all random drops, and even the named epic drops had random stats.
To put it in perspective, there were about 30 different random attributes. All builds require 4-5 of these attibutes.
str/dex/int depending on class
If you dont have ALL of these abilities on most of your gear, you simply cannot complete end-game content.
If there is one thing I'm becoming exceptionally tired of, where RPGs are concerned, it is this scenario where the end game is the only thing anyone talks about. I've spent some time on the Borderlands 2 forums. That has a fairly long levelling game. Not months long, but getting to UVHM took me probably a month, playing on and off; I've got about 270 hours logged on Steam, now.
In said forums, however, whenever newbies tried to get advice about weapons, the only thing that anyone would answer them with, was information about end-game named uniques or legendaries, for the most part. There was precious little info offered about the manufacturing corporations, the different elemental damage types, etc; stuff that people needed to know for the whole game. There was also the usual bullshit insistence that some character builds were not "viable," for end game content, when I've been going through UVHM with a Survival Commando, (probably the class/spec combo which attracts the largest amount of shit) with no problem at all.
We have a couple of different problems here. The main one, is that forums in particular, and possibly these games in general, end up infested with a certain type of person who does not actually enjoy the game, and who is not there purely in order to play said game, but is motivated purely by a desire to be viewed as good at the game, for the purposes of ego gratification. It got to the point in the WoW forums where people were admitting that fairly openly.
The second problem, as an extension of the first, is that you have a very large number of frankly terrible players, in terms of their actual level of ability, who criticise the developers for adding talent trees to the game which are supposedly not "viable;" when again, said trees are usually fine. It will be the players themselves who suck, not the trees.
I'm not saying that D3 in particular was not an attrocious game; although I haven't played it myself, everyone I've spoken to about it, has consistently said it was terrible. At the same time, however, it needs to be acknowledged that the above problems do occur; particularly on forums.