I'm actually pretty surprised that I find myself with a desire to write this post. There are some sort of basic assumptions about difficulty in WoW that I thought were universal, but it turns out they're not. I shouldn't really be surprised, of course. There does not exist an opinion about this game that you can't find someone to disagree with. Up to and including the opinion that it is in fact a game. There are a number of ways to disagree with that, but then I'm wondering off-course.
Alright, difficulty. To begin with, I think that any given encounter or activity or place in the game has a learning curve. These curves vary in steepness. Lost City of the Tol'vir, for example, is easier to learn than pre-nerf heroic Stonecore, where all the bosses had a number of instant-kill mechanics. Instant-kill things are tougher to learn because if you miss them once you're simply dead, and you don't get another look at the warning until the party wipes and comes back or the next time you do the instance. The important part I'm getting at here, though, is that some encounters are more difficult to learn than others.
I also think that WoW is entirely about teamwork. Classes are designed to create synergies, mechanics require correct relative positioning, battlegrounds and arenas depend on communication and cooperation, on and on. Right? I don't think any of this should be controversial.
If you accept the previous two statements, however, I think you're pretty much required to accept the logical conclusions that extend from them. One of those conclusions is that there is no such thing in the game as something that is hard or something that is easy. The Sinestra encounter one-shot farm content for many guilds at this point. So it's easy for them, but that doesn't mean it's just plain old easy. Converseley, even though most guilds aren't even going to pull Sinestra, that doesn't mean it's difficult. It's just difficult for them.
This might seem like froofy, new-age nonsense, but I really don't think it is. I think an encounter's difficulty is entirely dependent on the group that's engaging with it. A guild group on vent with a lot of experience working together can pretty believably get a new ZA bear on the first run through the place, as long as they've all read up on things beforehand. A group assembled of people that don't know each other through the RDF system can wipe several times in ZA even if everyone in that group has cleared the instance a few times before. The encounters are not inherently easy or inherently difficult. The most that can be said about them is that their learning curves may be steeper or shallower depending on the group making the pull.
In turn, this is why I don't really feel like it makes a lot of sense to upbraid your RDF party members as bads or terribles or whatever. Any particular person may be having trouble picking up or executing a particular thing in an encounter, but they're just a part of the group or raid. A sufficiently practiced group can carry pretty much anyone through pretty much any encounter, as illustrated by guilds doing things like selling achievement drakes (or achievement bears from the raid version of ZA a couple expansions ago).
Basically, if you're calling your party members bads, you're saying you're not good enough to cover for them. If you feel like someone just isn't getting something, you can vote-kick, sure. But going on about how stuff is "easy" and not at all like other stuff that's "hard" just gets you into an endless pissing match that, unless you're in a top-100 guild, you're not going to win. I've seen people that (like me!) still haven't cleared T11 normal calling people bad. Or people calling their teammates in BGs bads and terribles even though, on later armorying, their arena/rated BG teams are in that 1500-1600 range.
As long as you can keep in mind that no particular thing is "easy" and other things are "hard," you should hopefully have an easier time reigning in those impulses to shout at your teammates. Unless of course they're being little pricks, which is the one thing that's guaranteed to get me shouting at someone.