I would caution, first and foremost, against misconstruing my previous post as a flat defense of rude behavior from any source. My aim was to avoid that by emphasizing that this isn't an issue that's the fault of any particular role, but that we all collectively need to think less about getting ours and ours alone, and think more about making a team effort regardless of who we're paired with in whatever content.
You've got to consider that tanking is incredibly difficult for a number of reasons. Impatience from the DPS, the healer, etc. isn't going to help a situation where a tank is new, or even if they're not exactly new, inexperienced with a particular dungeon or raid. Tanks are hard to come by, yeah, and good tanks moreso - but the fact is that you're rarely given leeway on a tank job to learn a dungeon or a trial fight unless you were there when the content was released and brand new. For as many people as are willing to be patient and supportive of a new tank, there are just as many inclined to brook no quarter, and enough harassment and impatience will turn people off to tanking, or drive them from the game entirely. Hence, we wind up with fewer people compelled to put in the massive amount of time and effort needed to become proficient at all the content they might face, and a greater number who are taking their first forays into the job, only to get hammered down for not knowing everything from the get-go.
At the same time, I understand where you're coming from. I'm not looking to suggest that tank players can't be and aren't just as likely to be rude and toxic to their DPS and healers (as evidenced by some of the above posts), but that again comes back to this issue of all of us needing to expend less effort on blaming others and instead exercise more restraint, and being more willing to offer advice and support. You are, of course, going to run into people who don't and won't give a damn about any advice offered to them, but my view is that you handle those people on a case-by-case basis after the fact, rather than making assumptions from the get-go.