Special circumstances call for different reactions. There are plenty of good or decent tanks too. It doesn't change the fact that a DPS that is reasonably geared can strip agro from even the best tanks if they really wanted to.
Paying attention to how much agro you are drawing from your actions is still everyone's responsibility. Unlike most other MMOs, this is clearly shown both on the party display for the active target, and has a general indicator next to every other target. A good DPS will meter their abilities early on so that the tank has enough time to build up threat before using damage buffs or off-GCD attacks. Unlike most other MMOs, the tank (both paladin and warrior) lacks any ability which is an instant re-capture of agro, so must maintain it throughout the fight. There is no taunt spam ability other than the combo attacks made by the tank, and even that is limited by TP pool. The fact that a fully leveled legacy player does not understand this probably explains your low opinion of tanks. If the tank is geared lower than the DPS, the DPS should recognize this and either accept that they are responsible for staying alive through whatever mobs they pull, or should adjust their rotation accordingly. Of course a fully geared DPS will always strip agro from a newly AF 50 tank, even with standard rotations... Use your brain and adjust as needed so the run goes over smoothly instead of trying to pull out your E-peen, seeing how high you can crit or how quickly you can kill while tanking mobs; your run may be a little longer, but you'll look less like a douche.
Most of the "not going to heal you" stuff I've seen has only occurred in lower level instances, and usually in relation to DPS who intentionally pull mobs before the tank, or who consistently attack other targets than the tank is targeting. With the exception of the occasional Fate-bred DPS, I have not seen this become an issue post 40. Those people, the ones I am specifically talking about here, are the ones that really do need to learn how the game works and how their class contributes to the standard party. Keeping them alive while the tank exhausts everything trying to get back agro, then berating the tank for not being overgeared, or losing agro because someone decided to activate raging strikes and go through a full burn rotation at the beginning of the fight is not helping the matter.