I also really don't get the focus the article places on the DM. I mean, obviously the DM gets to approve classes and races and such, but should every wizard player really have to prepare a sales pitch for his preferred casting method?
Unlike the other direct posts I have seen, I'm going to say YES.
Yes, the lowly player will have to petition the DM for a spellcasting system which the game/campaign/setting is not by in large using already. That is a good thing.
If I am running my game and I decide that everyone should be using the wizard mechanic, or the sorcerer mechanic but not the warlock; then you come and ask me to play a warlock I can say no. If you want to pitch me on why you playing a warlock is a good idea for your character/concept then that is fine and I may then allow it. But as a default if I have already established that warlock mechanics don't exist then it is a good thing I am not suddenly expected to allow them just because WotC released another book.
To be fair, it has always been this way, that's why I disallow psionics, incarnum and book of 9 swords in my 3e games. If you want to play anything from those books you have to come up with a damned good sales pitch.
The flip side of this of course is that not everyone is required to use the wizard or sorcerer mechanics only. If the game is opened up enough that a larger variety of choices is available that just gives us so many more tools in the box. This is akin to 4e classic vs. 4e classic + essentials; both of which don't allow 3e vancian magic.
The typical reaction to "scratch out 'ranger', write 'fighter' instead" was usually along the lines of: "But I want to play a fighter, not a renamed ranger."
I think the problem with this is that when I want to play a fighter, I want that fighter class to allow me to play it my way, instead of having to play another class and still not get 100% of what I wanted in the first place.
Meaning, if I have to play a ranger with a scratched out name and written in fighter then I am significantly closer to the character that the party ranger (without the rewritten name) than I am to the party fighter (without the rewritten name.. and a bow).
That is the benefit I see with Mearls' most recent idea. That the mechanics of spellcasting, not the fluff or bonuses or anything else, but the HOW TO of casting is independent of the class. I like that, surprisingly. They just need to make sure to follow it through. It is actually exactly what I've been proposing since the Sorcerer and Warlock showed up, that they need to divorce At-Wills from the class and make it a universal option. To be entirely accurate it is the same thing I've been saying about CS dice too.
Okay. But 'spell memorization' is itself a spellcasting mechanic which can presumably be switched out for something else? Or are you taking it as given that wizards always memorize spells in some fashion?
The more I read the column, the more I think he means that switching out spellcasting is limited to the wizard, and perhaps the cleric. Even so, that leaves many questions.
It depends on how they end up doing this thing, I think the first couple of rounds will likely only affect the wizard, but if done correctly then I could easily see it diverging.
We could (and I think should) have a default/core/generic/universal wizard who has a spellbook. Then we could have a wild mage, then an elementalist, or evoker, perhaps an illusionist or summoner. Basically make wizard the umbrella name with a default mode of spellbook and default array, then open up the options to include all the different kinds of wizards that could still fall under that base. As long as these other wizards still need to plan ahead, are relatively weak in hand to hand and use magic from sources outside of themselves I think they all work relatively well under the wizard title.
With Mearls' idea they COULD all with vancian, at-wills or whatever. The mechanics are a different layer to tell us how the magic works in a specific world.
On top of that I would have the sorcerer be the one who draws power from inside, from their bloodline (though I hate that term). They may be more powerful in bursts or with reserves always available but lack the breadth and width of the wizard. Again, that is completely separate to what kind of sorcerer they are and separate again to the magic system being used.
Of course the reason I like this idea the most is because I dislike the warlock we currently have and I dislike the sorcerer with armor we currently have. I'm also not a fan of breath weapons

. But again, if done well that will just be minor aspects of the whole.
For warlocks, if they do intend to pursue this pact magic stuff then they should go full out. Give us different variations that ARE NOT just different species - ie. Fire, ice, darkness. They could require big elaborate rituals, or that the benefactor live inside the warlocks body or that their magic only works at specific times. Again, independent from the mechanics being used.
And yes shadow, I can see them applying this to clerics/divine casters too.
Better yet I can see them applying the mechanics separate from the classes. Allowing any character class to benefit from the magics, if done broadly enough. It allows options where at-wills or spell-slots could be given to a plain fighter in the right (preferably in game) circumstances.