Nah, I get the op point.
If you've played a decent dps ( god forbid me saying I'm good at that game EVER, damn I suck...), you at least once came into a group where tank was pulling, 1 (or sometime way more) mobs are not hit, turn their face to the healer and start bashing your lifeguard. You jump those mobs (most of the time they would be your normal target anyway), get aggro from those too many angry beasts, then die, while your group barely survive.
And then, while you still feel a little heroic for your noble sacrifice, while the healer is whispering thx, trying not to offend the tank, the big guy starts ************ about your bad focusing, calls you a noob and leave. Seen this scenario at least 5 times this week (admittedly in lvling ops, not hms).
Tank role as been pretty well described, but here is something not said enough :
DPS, your first priority is always the stuff that is trying to hit your healer, unless the healer can take care of himself. When tank picks aggro back, you go back to your priority list : weaks, strongs, elites (unless a specific fight works otherwise).
Now if the primadona tanks can undertand that (aka : sometimes they are not in their best shape and they make some mstakes to), we can all go back to bashing ninja pullers, dps that can't count to 2 before unloading everything in their arsenal and the ones that are just gonna hit like ******* wathever is closer to them.