I had a guy give me a bunch of high-end gear for my "mage" on Diablo 3 and still did terribly. Granted it was the first time I played in 3 or 4 months, but couldn't get my spell rotation and timings down to save my life.
I then pvped a friend who was a "hunter" with half my gear and easily killed me off.
So wasn't sure if this was going to be the same way or not. WoW on the other hand is more what you described. You have a realtively easy spell rotation and as long as you don't mess up the basic mechanics of the boss fight, you just need the gear to be able to dps fast enough.
It all comes down to what you end up playing and if you are paying attention to what is going on.
If I end up getting 20 stacks of bleeding on myself i'm not going make it very far. So most carry a flask to remove bleeding just for that alone.
The build I linked to the toon i'm playing now is not very complex. I mostly clear trash with one skill called Molten Strike. If i'm dealing with a hard pack i'll use blood rage which is an instant cast for an extra boost. Once I get to a boss or something even harder i'll drop an Ancestral Protector totem and keeping up my Flasks till they run out of changes.
There are a lot of detail guides out there on different ways to play a class. The hardest part is finding the right passive skill tree points, what skills to put in your gear, and what gear to use itself.
There are some out there that only create build guides and they have been for almost as long as the game has been around. Theory crafting is there fun in this game.