Hussar's comment are interesting as usual.
Hussar said:
3e's flavour was locked incredibly tightly into the rules.
I usually think that 3e allows for very good flexibility in the flavor, as long as you don't try to step out of some of the core assumptions, such as the power of spellcasters or the magic item quantity. On a smaller scale, it is instead quite easy to write up or modify classes, feats, spells etc because it's quite modular.
But indeed there is some sort of "lock" between flavor and rules. Just to make a very small example, weapons are designed in a certain way so that the game assumes that every sword has a 19-20/x2 critical, and every axe has 20/x3. The type of critical could be a tactical choice for your character, while the weapon "image" could be a flavor choice. The two choices are locked, you cannot make one without locking yourself with the other. You can of course house rule something, but it's like the ruleset assumes the lock is good. In OD&D all weapons did the same damage, so the flavor choice was free: the downside is that there is no tactical option at all of course, but still the feeling it gives you is that there is no lock.
Hussar said:
What 3e didn't have in core was any actual flavor to go with all that crunch.
This can be either good or bad depending on how you ask. Some gamers want flavor in the core, and despise a RPG which is just a "tool", they want a world ready to be played. Others do not want the flavor defined, but exactly want only a tool to which they can apply their favourite flavor (homebrew or from an existing setting).
Personally, I think that 3e fell in the middle. Some common ground of flavor was there, in the choice of races, classes, spells and magic items. But these are so modular that it's easy to remove those which don't fit with your favourite flavor, and add new ones. OA you mentioned is just a good example, and IMHO Rokugan is even better: they removed classes which don't fit (cleric, druid, paladin, bard, wizard), adjusted those which were almost ok (barbarian, ranger), kept some untouched (monk, sorcerer, fighter, rogue), and added new ones (shugenja, inkyo, ninja, samurai, courtier). That is IMHO a nice feature of the system.
Hussar said:
So, it looked like you could do all these weird and wonderfully different campaigns with D&D 3e right out of the box. Until, that is, you actually tried to run campaigns which deviated from baseline norms. Suddenly vast swaths of problems crop up. Go too low on magic items and casters dominate. Lower the powers of casters and you suddenly make the game so lethal at higher levels that it's unplayable because you don't have healers.
Indeed, you cannot easily remove those few basic assumptions of 3e. I would have wanted a low-magic-items game sometimes, but if I just lower them I suddenly have difficult problems in choosing monsters for the adventures. If someone want a world where spellcasters don't have truly amazing spells (the higher levels), you easily end up with spellcasters being near-useless when the game goes high in level.
Fact it, I don't know yet any ruleset that allows flexibility to such extreme degree that you can really create anything between no-magic and anime. I believe that at a certain point every system is limited to its basic assumptions, and if you want to go beyond you really just have to switch to another system.