How does this structure prevent unfamiliar ideas? A subclass can be just as new and unfamiliar as a regular class.
It's a matter of degree. If you have five characters who are all fighters but different subclasses they will be more alike than unalike.
If they were all martial melee classes they'll be almost completely unlike. (Or the classes will be redundant.) So describing the classes takes more effort.
And what is wrong with "unfamiliar" ideas? If you don't like a new class, nothing forces you to use it.
And nothing is forcing you to use subclasses.
But this actually highlights another benefit of subclasses. New classes mean adding something to the game. New mechanics to learn, new content to add to the world, etc. More work for the DM. If not buying a new book means you don't need to add things, it's easier to skip the book. If the options just expand on content you're already using, classes and races that are already a part of the game, it's easier to work into the game. Which makes Accessories more attractive, as their expansions not ad-ons.
I also want to point out that I'm fine with the whole subclass thing for the most part. I think it's fine to make assassins and shadowdancers subclasses of rogue, etc. The only thing I'm strongly against is putting every type of non-divine magic user under one class - the mage. It's just too much. All of the other classes are pretty consistent in the amount of options and concepts they cover (except for the barbarian, which IMO should be a fighter subclass). But then you have the mage, which includes every type of wizard, sorcerer, warlock, witch, artificer and even psionics all under one class, potentially making that one class as large as all of the other classes combined. It just doesn't fit with the rest of the classes in this design scheme, at all. It's like you have a lineup of meerkats, and then throw in a rhinocerous. The "mage" is just too broad a concept, and needs to be broken down. I can live with the sorcerer being together with the wizard, as they traditionally used the exact same spells anyway. But the warlock and psion would work far better as their own classes.
The Mage Issue is non-representational of this issue. That's a whole other discussion. The Mage Issue is really about turning formerly core classes into subclasses, while much of this subclass discussion focuses on reducing splatbooks classes into subclasses.
This isn't an either/or issue. You're allowed to like the idea and focus on subclasses over new classes and still think they're going too far. Personally, I think adding psions to the mix is too much. But I still like the focus on fewer classes.
When a subclass can have a totally different casting method than its parent class, there's not really much familiarity there. You still have to learn about the new subclass's methods, spells, etc. Saying its a class or a subclass makes no difference as far as that goes. It's merely a matter of organization. I can put the warlock in the same filing cabinet as the wizard, or a different one. But it's still a warlock.
The only way that what you're saying would be right is if every mage subclass were forced to use similar casting mechanics and spells to the regular wizard. But I would absolutely hate that because it would take away everything that made warlocks and psions special, IMO.
This is irrelevant.
They've already said (multiple times over the past year) that they will have alternate spellcasting. So, regardless of class, you can use Vancian or spell points, or powers. So if someone wants to play an arcane caster they don't have to pick a side class if they hate Vancian casting.
What will seperate the warlock subclass from the wizard subclass will likely be mechanics unrelated to casting.
I disagree. There's absolutely nothing preventing a subclass from being overpowered if the abilities it gets are more powerful than what other subclasses get.
True, there's nothing stopping a subclass from being OP. But that wasn't what I was saying. I was saying that it's
easier to balance a subclass than a full class. Because it's simply less work to balance abilities gained over 3-6 levels rather than powers gained over 20 levels.
In addition to having fewer powers, you can also compare it to other subclasses, rather than trying to balance against every class in the game. As you already have the relative power of the class balanced.
Again, I disagree. If something is introduced in a later book, it makes no difference whether or not its a class or subclass as far as support goes. In either case, it may never be mentioned in a future supplement ever again.
Actually, it does.
If it's a class then you either have support or not. However, if it's a subclass then a lot of the content created for other subclasses might apply.
If there's a separate shadowdancer you might not see feats, items, and the like that works with that class. If the shadowdancer is a rogue subclass then options for the rogue -both past and future- might work for the subclass. And content that is created with the shadowdancer in mind might still work with other rogue subclasses.
What you're saying about feats and the like is also not necessarily true. A feat designed for a vancian wizard may simply not work for an at-will warlock or a spell point psion. Metamagic type feats that require spell slots are a good example of this.
Again, with alternate spell systems this is moot. You could okay a wizard with powers or a warlock with Vancian spells.
The trick is phrasing the rules module to work with feats.