Recent threads about Vancian casting have me wondering: how much diversity do various magic systems out there actually encourage in practice?

I can think of two different things out might be valuable to measure: build diversity, and play diversity.

Let's look at MtG as an example.

The vast library of spells allows for nearly limitless build variety. And, in casual games, that variety shows. However, in top tier competitive games, the metagame usually limits players to only a few valid builds.

MtG makes up for this with various formats, like draft and sealed, where you are all but forced to play with otherwise unplayable, suboptimal "trap" options. Formats like Commander or 2-headed giant where the relative values of certain cards change. And formats like community deck, where the deck's builder can sculpt the metagame.

In play, some decks are very… formulaic. Probably most are. They play one specific way, with little variance. The randomization via shuffling of the deck has some effect, but, depending on the deck, it's often just "works or doesn't" rather than any significant amount of "works differently".

So, back to RPGs - what various RPG magic systems are there, and how much diversity do they encourage, in build and in play?

I'm guessing, at a minimum, we'll want to look at D&D (3e, 2e, maybe others), WoD Mage, GURPS, Ars Magica, ShadowRun… what else? I'll probably throw Marvel facerip and Mutants and Masterminds into the mix. Any other games whose systems of magic should be considered?