The biggest story from Blizzcon was the reveal of a Diablo mobile game, and the subsequent backlash from the crowd.
Personally, I think the backlash is a little excessive. Like many, I was disappointed that Diablo IV wasn't revealed. But if Blizzard wants to make a mobile game, to explore that space, then a streamlined Diablo III is probably the best fit. I don't play games on my phone, but--assuming the payment scheme is sensible, which is a big assumption--I might try this one out.
Though I suppose the backlash is just par for the course in the current age. Everything is pushed to the extremes. Something is either a 10/10 or a 1/10; the best or the worst; love or hate. There's no room for indifference, mild curiosity, or a wait-and-see approach.
I think that Blizzard made three mistakes here. First, they should not have announced the announcement. They should have just not said anything about Diablo like they normally do. Instead they explicitly said that there would be reveals at Blizzcon, and that got the hype train out of control.
Second, Blizzard should have been explicit about the payment model. I believe the main reason mobile games are disliked is that the micro-transactions are widely seen as annoying at best and predatory at worst. I would love to see Blizzard use their clout and establish a sensible price for a mobile game. The fact that they are not saying anything about payment models gives rise to suspicions that they're going to go down the predatory micro-transaction route.
Finally, Blizzard sends mixed messages about fan response. For example, in the WoW presentation, they talked about how the "Remove shoulders for Saurfang" had a big impact on the team and led to trying to make choices that matter (SWTOR players winced here). But saying things like this just encourages fans to overreact and start "movements" for every little thing. Blizzard would do far better to name drop people who make polite, reasoned arguments on the forums.
Basically, if the squeaky wheel gets the grease, don't be surprised when all the wheels suddenly start screaming. You get more of the behavior you reward or subsidize.