This might end up being somewhat blasphemous, but I have found that tanking ends up taking less overall brianpower versus raid leading, in terms of multitasking the raid. The raid leader has certain responsibilities like deciding who gets a brezz, calling things out on vent, calling for wipes, things like that. As a tank, you're generally responsible for 1 thing at a time, either positioning, a taunt, your health for a cooldown, or an interrupt. At most you are doing 2 at the same time, maybe a personal cooldown while kiting. You'll rarely ever need to worry about positioning while interrupting, for example. In other words, while your job is important, and a screw-up will lead to a wipe, you generally only have one thing to do at a time. It does get more complicated on heroic modes, but not overly so.

DPS, on the other hand, requires strict timing and precision, and tracking of multiple things such as buffs, debuffs, procs, DOTs, DPS cooldowns, mana/energy regen cooldowns, as well as switching targets, switching between AoE and single target rotations. The difference between a top fire mage and a mediocre one can simply be a matter of tracking dots most effectively and timing a combustion during an add phase.

What I'm saying is that while tanking overall is the more important job, tanks generally have less to think about during the battle, focusing on one or two things, versus dps who, if they are min/maxing, have much more on their screen they are looking at. This frees up brain space for multitasking things like raid leading, deciding who gets brezzes, calling for wipes, etc.