WoW expansion

Been a warlock and DK theorycrafter/number cruncher for some years, I'm full nerd about shit like that so I can provide some facts:

For melee, raw ability damage, as in unbuffed by main stats like strength/agility/int as well as secondary ones, is always inherently higher than casters. However, there is something called scaling, which means by increasing certain stats, you get increased ability damage, and for some unknown reason Blizzard always provides better scaling (ability coefficients) for caster classes, no exceptions. So, the better gear you get, the more stats you get, leading in better damage scaling of your abilities.

Conclusion: melee at the start of expansions are almost always better than casters, but once gear gets one or two (tops) tiers better, casters begin to break away, to the point where melee can't keep up anymore because they don't scale as well, leading to caster nerfs (but not always, if you look at mages end-Cata or Warlocks end-MoP).

Evidence: every single WoW expansion ever.

edit: this is based on the notion of computer simulated scenarios, in reality no one plays a class at its 100%, but there is a certain imbalance/discrepancy between classes (10-100% in some cases) as far as damage output goes even in reality (for the reasons stated above).