Actually I don't see where your comments about aggro mods not being used comes from, considering that blizzard had to actually legitimate them by designing encounters specifically around their use (bad choice, they should have just banned them).
Maybe your personal guild doesn't use them (or maybe people just say that they don't), but that doesn't mean that they aren't widely used.
If the gear separation is too great, you mitigate it, instead of legitimizing tools that contribute turning one of the few relatively challenging aspects of the game (tanking) into a matter of just following the instructions on screen.
But of course Blizzard didn't mitigate the gear separation, know why? Because it makes them money, keeping people hooked on the game.
And that's the big problem with the challenge in every MMORPG. WoW included.
MMORPGs are based on people spending money on them on a continuated base. Many, faced with content that they cannot overcome easily, simply quit and stop paying.
This was the main reason that killed warhammer online. The game was massively based on faction-related PvP, and people that found that their faction couldn't overcome the opposition simply quit. Then the opposite faction found themselves without people to play against, and quit out of boredom. Then the first faction started dominating because more people quit on the second because of boredom, triggering more ragequits, in an endless loop of ragequitting.
This is why it's in any MMORPG company's best interest to keep content relatively unchallenging. Between MMORPG players, people that want an actual challenge are much less than those that just want to "be the hero". WoW is no exception, and if you think that it's any more challenging than the average of other MMORPGs, you kid yourself.
Some encounters in WoW have a relatively decent level of challenge, like the twin emperors for instance, but more or less every MMORPG has it's ration of those, some high tier raids in Age of Conan for instance (at least before funcom dumbed down the combo mechanics), Pretty much every COP encounter in FFXI, most Tomb Kings bosses in Warhammer online (Mythic really went on the cruel side with those, erroneously thinking that people WANTED a challenge, just triggering more ragequits instead). There are a few in every MMORPG out there. And mind you, I say more challenging than the average, but they're still fairly unchallenging compared to other genres.
By the way, I'm not saying it's a bad thing. I enjoy relaxed gameplay quite often and you won't ever be able to quote me saying that WoW's problem is that it's easy (while I believe it is, that's not the problem, besides the fact that it's extreme easyness during the leveling up process tends to attract a crowd I don't like to play with), and just go to other games (racing simulators, flight simulators, strategy games and so forth) when I want challenging content.
PS:My facts are always verifiable. Some just don't like to hear them, and prefer to deny.
The fact that you don't enjoy or prefer something doesn't mean everyone doesn't, or even the majority. Most final fantasy players enjoy storylines, that's why they play FF in the first place, and that's obviously one of the main "rewards" a final fantasy game does and should offer.