I think we can safely agree that 'Gimmicks' encompases a wide range of things. On the one hand, you can point to Anub and say "there are exactly two of four tanks who are expressedly forbidden the off-tanking role, this is a gimmick." On the other hand you might say "there is a lot of spell damage in this encounter, resistance gear makes this a gimmick encounter." Or you might even say "Icebound fortitude made this encounter a gimmick given the very strong burst mechanic on a 1min timer."
In each case you've got a strong argument that the encounter is indeed a "gimmick." But, for the purposes of wotlk in 3.3 and beyond we have to more or less stick with the first one. Resistance gear has been with us forever, and it crops up now and then, we can safely assume every tank will have such gear avalible to them. Over-tuned cooldowns are largely a thing of the past (even the 'cooldown' portion of AD isn't really that over-tuned, it's the passive mitigaiton part that is) so you can't really argue that anymore either.
To make a long story short: blizzard now has to avoid decidedly fewer things in order to keep an encounter from going to "gimmick" status. (Note: i'm going to avoid the vehicle argument because that effects all tanks equally) Until cataclysm they must avoid: Encounters that heavily favor block mechanics and possibly a few other niches that could easily make a minority of tanks heavily preferred.
To tie this into the general theme of the thread: So long as every tank is more concerned with their own personal execution of the encounter than the mitigation, utility, avoidance, or block their class can deliver, blizzard has done a good job.