There's really no mystery here about why there's so much fan rage around Diablo 3. It's pretty much been a perfect storm of reasons:
First, we have an extremely popular game that fans have been waiting for 10+ years for a sequel to, in a game genre that has relatively few entries, so the fans are quite starved for it.
Next, the pre-release game hype started literally YEARS before the game was actually released, getting fans excited, but with little to see except a few screenshots. So they focus on what they CAN see: the look/art style of the game, which is somewhat of a change from the past.
Closer to release, Blizzard announced the "Real Money Auction House" (RMAH), splitting the fan base into those thinking "Yeah, I'm going to make money selling stuff from playing Diablo!" and those worried about the negative impacts the RMAH will have on the game (such as for the item economy balance and restrictions for greater online security).
At the same time, fans are informed that Diablo 3 will be online-only, even for people just wanting to play single player. This is clearly the first big consequence from Blizzard adding the RMAH, and this requirement negatively impacts or totally shuts out:
- those with laggy/flaky/poor net connections
- those traveling or in the military that cant get online
- those that want to use game mods
- those that like to use cheats in single layer
- those who dislike always-on DRM
So the online-only requirement splits fans into 3 groups: 1) those that didn't care (because they'd likely be online/multiplayer anyway), 2) those that decided not to buy it and would likely prefer to see Blizzard's new profit model fail rather than spread to force more single player games online, and 3) those that cared but were starved badly enough for D3 to buy it anyway and give it a chance.
Then, the game comes out, amidst a nightmare of networking issues. Issues that would not have existed or would have been far less impactful if all the single player people hadn't been forced to play online. All the group 2 people revelled in their schadenfreude and said "I told you so". And the group 3 people (who were dubious about the situation but still laid out their cash realized that their fears had been justified and they've been badly let down by Blizzard) start to loudly complain.
Then fans start to notice that the item drop rewards are noticeably poorer than in the previous games, particularly if you aren't playing at the highest difficulty level, which requires many hours of gameplay to unlock. Cue the conspiracy theories about the drop quality being tuned to support the RMAH.
And all of this fan crankiness is before even considering the actual gameplay, which has its inevitable detractors.