The way I see it, something should be a subclass when it is just a minor variant of the main class and still shares the basic features of that class. A specialist wizard is still a wizard, as is a wizard of high sorcery, a dark sun defiler, etc. A shaman is just a variant druid. A totem barbarian is still a barbarian. A blackguard is just a paladin that serves dark powers. But is a warlock or psion really just a slightly different wizard? Hardly. You could say that about sorcerers, but not warlocks and especially not psions. They have pretty much nothing in common except that they use magic. A druid and cleric have far more in common than a wizard and psion do!
The main thing that should determine whether or not something should be a class or subclass is whether or not its concept is broad enough that it could have subclasses of its own. Warlocks and Psions clearly fit into that category. Warlocks have a variety of pacts, and as in 4e, they could include the binder as well. Psions have a variety of disciplines, not unlike specialty schools for wizards, plus the wilder and the other psionic classes introduced in 4e, like the ardent. Even sorcerers could very easily be their own class, with a variety of different bloodlines. Just because they were effectively just wizards that didn't prepare spells before doesn't mean they couldn't be designed well as their own class now.
I agree with everything.
I think eventually the problem is that each of us has a different image of these classes, what they truly are.
For example, sometimes I feel like a Druid could really be a specialist Cleric of an old faith that doesn't worship gods. That could be the case IF you don't frame all Clerics to be "deity servants". But if you frame the Cleric class to be "she who gets magic powers from a deity" then Druid doesn't fit. I would be totally fine if Cleric was rebranded more generically as "religious figure with mystical powers" and then it would include the traditional single-deity cleric, the druid, servants of a whole pantheon, saints/mystics who serve a philosophical concept and new things. That superclass could also possibly include the Warlock, with which it would have in common the outside source of magic power (i.e. both Clerics and Warlocks don't study/understand their spells, and get them because of a "do-ut-des" with some otherwordly power); the difference is in the spell list and the old arcane/divine distinction that can be always argued*. The Cleric superclass could also have a feature similar to Mage Wizardry/Sorcery at 1st level, that would change the basic assumptions on how and why you get your powers, how they work, and what spell list you follow.
There are many ways to implement these kind of stuff, and there is always benefits and downsides, hence criticism, no matter what WotC does, but they have to pick something...
*some people see Arcane & Divine as very different things, some people see them as a feeble distinction because after all they are both magic, and besides traditionally having very different spell lists, always used the same mechanic (except details like armor interference); the same thing happens with Arcane & Psionics, there are gamers who strongly feel they are the same thing, and others who can't tolerate mixing them up
Anyway, the point you make about "concept breadth" is quite important. They definitely hinted that they want to make several Sorcerer Bloodlines at least, and Wizard subclasses (Arcane Traditions) are undoubtedly easier to proliferate than martial classes subclasses, so I would expect that even just the Wizard version of Mage will spawn the largest number of subclasses (or contend the record with Cleric domains).
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I've said before that IMHO lumping all arcane casters under one class accomplishes nothing, it has no benefit. It's almost exclusively a different presentation of the same thing. Now they have to really think of what the two alternative presentations will deliver in terms of feeling, when someone cracks the PHB open...
1) Separate classes would look like each of them (Wizard, Sorcerer, Warlock) gets an equal entry in the PHB, a picture, an iconic character (with the Basic game default subclass, e.g. Generalist Wizard, Draconic Sorcerer...), and a character advancement table that includes spells per day just like for the Cleric, Druid, Paladin, Ranger.
2) One class to rule them all would look like its entry takes 3 times larger space in the PHB compared to other base classes, requires multiple iconic characters OR alternatively forgoes everybody except an iconic Wizard. The player will have to look at 2 character advancement tables simultaneously, one for the non-spells part, and another which is different each variant and has slots per level per day for Wizards, spell points for Sorcerers etc.