I don't play WoW atm, and haven't for a while so I know nothing about how Arcane Mages work right now. So I'm going to use MoP Warlocks as an example of Blizzard giving players complete control over their damage, leading to OP damage.

Demonology+Unerring Vision of Lei Shen: Doom crits spawns an imp. In Throne of Thunder there's a trinket that gives you 100% crit for a short while. Due to snapshotting players could abuse that and cast Doom only when the 100% crit proc was up. Making sure you spawn an imp on every Doom tick. The more targets you could keep this on the better, and there was no RNG to it. The worst that could happen is that your trinket doesn't proc right on the pull and you miss 1-2 imps but over the course of the fight it's a huge damage boost regardless. Blizzard didn't fix this until SoO because they wanted you to use newer trinkets instead of an old OP one. Except they made a new OP trinket!

Affliction+Black Blood of Y'shaarj: This is why everyone hated on Warlocks in MoP. One easily abusable mechanic, and Blizzards stupidity to add one of the most powerful trinkets ever into the game. The trinket procs and gives you an insane amount of intellect. Refresh your dots during it and your dps goes through the roof. On a fight like Protectors(3 targets up at all times) you can keep superpowered dots up for the entire fight because of Soul Swap.

So the problem here was player control. You have snapshotting and you know when trinkets are going to be ready. Doing it correctly will put you miles ahead of someone who doesn't. I don't know why but Blizzard saw this as a problem. So they removed snapshotting and made dots dynamic. So you don't control when to refresh them, you just keep them up. Then they made trinkets unreliable so you don't know when they're going to proc. Even less control. If dots were dynamic and trinkets completely rng, Warlocks would not have been as dominant in MoP.

The only class I've raided with in WoD was Hunter. I had no control over my dps at all. All I did was press Chimaera Shot and Aimed Shot so I don't cap focus, regardless of procs. My dps was all about "did Chim crit? multistrike? twice? did multistrikes crit?". My dps from pull to pull varies a lot because of randomness, whereas it would always be high for a MoP Warlock because I had control.