The good tank will not stand in shit, take average amounts of damage, and do average damage. The great tank will not stand in shit, do excellent damage and rival if not surpass the dps (as of this expansion), will do a fuckton of healing, will move bosses in a very particular way (one that any decent melee dps will notice is a bonus rather than the erratic circles the majority of tanks go for) and can provide utility for the raid above almost anyone else.
I'm not admitting that tanking is easier than healing or dpsing because I completely disagree, and I've done both roles from TBC until end of Cata. "It'd be a crime to say otherwise" is completely your opinion and one I totally disagree with. Dps and healers have a way to compare themselves through World of Logs - tanks do not have that luxury.
To notice a good tank is something you see rather than read about or study, and you sure as hell know when you've found one. The obvious differences may well not be as large as it is for a healer or a dps but that is certainly not to say that the ability difference between those dps doing 5k and 500k, and the tanks doing 80k and 90k, is not the same distance.