HP is a problem, relative scarcity (yes yes I know they're "rares") is another since they die so fast. Melee unfriendly is the other big one I have. I said it before, but I like how they handled one of the rares by having it be a mini event so you know to get there. They should do that more often. One of the few things I liked about GW2 PVE.
I think part of this is just about expectations....like you said yourself, they're supposed to be "rares" and in the same sentence you said that's the issue. And honestly, compared to other MMO's, they can't really be called rares. They're more like uncommons.
WoW has just trained the playerbase that, except for Heroic raids and maybe some of "Normal" raiding, everything should be very easy.
So now a situation where it's not necessarily easy and that's due to competing with other players and breaking the "themepark" model WoW had gravitated to and that causes....stress?
I dunno. It's not an easy answer. WoW probably can't afford to break the themepark model, but if they don't try stuff that takes longer (or new stuff period), they'll stagnate pretty badly (imo).