i personally hated the cata arcane mage mana mangement game
Energy. Combo points don't count as a resource, they are a modifier; all CP using moves still require energy, unlike Chi. Energy is the most complex and a primary reason why rogues are so unpopular. Good rogues deal tons of damage and can be a real pain in pvp, but the energy system requires patience and doesn't facilitate spamming in most cases.
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Even if they were playing it "improperly" in Firelands they were topping the meter...
Keep in mind, "most complicated" is a relative measure. It doesn't require the system to be complicated in an absolute sense, just more complicated than the other systems... and I think Runes + Runic Power qualify on that count. If you disagree, feel free to explain why.
The discussion is also about resource systems, not class mechanics, which vary not only by class but by spec. If your point is that the way Unholy uses the resource system makes it simpler in practice, that doesn't change the fact that the resource system is more complicated as a system, it just means that it was implemented in a way that is not complicated to use.
What? Energy is one of the easiest resource systems. It's completely passive, and only requires you to use your regular attacks as the energy becomes available. DK runes is like having 3-6 energy bars, and that's not even taking into account rune power.
As you said yourself, the only thing the energy system requires is patience.