Sometimes complexity is a good thing, sometimes it isn’t. WoW has had issues with this concept since launch, though the recent expansion has really focused more on the increase of this concern.
When the game initially launched, the variable DPS loads were less based on gear and more on rotation. Certainly raid gear helped but resistances got you through. Rotations were key and a lot of classes simply sat out entire raids because they were non competitive. The rotations themselves were fairly simple too, 3-4 buttons.
Add on a few expansions and we get to Lich King with some additional mechanics (hit/armor penetration, expertise) and the distribution moved onto gear rather than actual rotations. Hunters topped DPS charts not because of their rotations (though it helped) but because they stacked armor penetration to absurd amounts. Most classes were at a 6 button rotation, heck even paladins had a meme about it. This was a simple time.
Cataclysm removed armor penetration and implemented Mastery – a stat with differing effects, per class and spec. Rogues either increased poison damage, increases off-hand attacks or increased finishing move damage. Some were better than others, some were completely ignored. Skill bloat, more talents, more stats brought out optimal rotations not only based on talent choices but on gear levels. At specific levels of mastery or crit, you changed your rotation for some classes. Some specs were optimal in area effect fights, some were mobile, some were burst. A single class might need to swap between them on each boss.
This added layer of complexity is confusing at first and frustrating later on. If I say you have 5 things to remember in a fight and they are all DPS related, how do you manage to move out of the fire, turn around, attack the adds and press the clicky too? This is why the Looking For Raid tool was so effective, it brought down the bar for DPS requirements to a point where people could press 1-2 buttons and get through content. You could do 50% less damage than you would in a heroic raid and succeed. Swap to the heroic raid though and each attack was required. Drop 5% from optimal and you were nearly assured a wipe.
I won’t argue for or against complex/simple systems – each has their place. What I will argue against is using both systems on the same target audience. Shamans have it fairly easy with 3 things to remember. Good players and bad ones tend to group near the same DPS numbers. Warlocks (as in the link) have 12-15 things to remember. This means that there is a huge difference between the bad ones and the good ones. This variance makes the class less attractive on the whole and specifically less attractive if the best played are still sub-optimal. If I have to press 10 buttons to do X DPS and a mage only has to press 5 and does more DPS than I do, why am I playing a warlock in the first place? Taking it a step further, if a hybrid class can heal and DPS at the same level as I can, why play a DPS only class?
It must be quite the challenge from the design point of view. You need to put in the right amount of complexity to make a class attractive (not boring), competitive (+/- 5%) yet not so overly complex that you need to practice 8 hours a day to come close to optimal.
Mists of Pandaria is taking an interesting approach of providing more diversity between classes – essentially expanding the issue while trying to simplify it. Warlocks are being practically re-written. Monks will heal by punching people in the face. If you’re only optimal while attacking, what happens when you can’t attack (a Rogue issue for years)? When do you say “the line is here, we will not cross it?”