Here's a lil' bit of rationalization (it's important to keep in mind what a class actually keeps when the alignment is violated. Barbarians, for instance, only loose rage, and bards don't loose anything):
Barbarians: Must be nonlawful. Why? Because their main class power -- the rage -- is used through a loss of control, a degeneration from order to mere instinct, an eradication of reason, ration, and codifying. There is no hierarchy or structure to the rage. It is, in fact, a breaking down of the hierarchies and structures that normally exist in the body and mind, to tap it's full potential without a concern for control or manipulation. A lawful person cannot destroy those social and natural bounds that tie them up. The barbarians are tapping adrenaline and raw emotional gusto, and, to do that, they have to destroy what keeps that in check. They have to eradicate their own internal structure. They have to rebel against the norms around them. It's impossible for anyone who doesn't like that rebellion against self(e.g.: anyone lawful) to even consider the destruction of norms that can power a rage.
I would accept a lawful barbarian if they explained that the rage came from some other source, but as it stands the rage, at least by default, is a greatly chaotic act -- you rebel against your body's own limits. The Ki Frenzy of the Sohei (OA) class is a good example of a lawful rage-type ability.
Bards: Must not be lawful. Why? Because of their versatility and mutability. A bard isn't just incapable of focus -- focus is an anathema to them. Having talents from a wide range of classes means that you must be mutable, must be changable, must be able to consider yourself simultaneously a healer and a fighter and a rogue. You can't subscribe to the rigid definitions of what you *should* be. Taken to an extreme, it's almost like the mythological gypsy view -- when opressed, when controlled, you sicken and die. You crave freedom to do what you will. If you are lawful, ordered, codified, you are, by nessecisity, focused. And bards cannot be focused in anything but their flexibility.
I would accept a lawful bard quite readily, actually, but that's one of the alignment restrictions I most strongly disagree with. If they can explain why their character is still concerned with being flexible and unfocused and rather...lazy...in skill selection, I can live with it.
Clerics: Fairly evident. If you don't agree with your god, he won't give you your powers.
Druid: As cleric, mostly, except they have to placate the power of nature, implying the "extremes" (LG, CG, CE, LE) are unnatural. Probably more true that druids have to, to some extent, be instinctive, without pondering the world too much. The instinct can be an ordered one (think of insects as a prime example of this, but also social animals like, say, merecats), the important thing is that it's an instinct.
Fighter: Described as anything, really, because the class carries little to no preconcieved archetypes.
Monk: Must be Lawful. Why? Largely for the same reason they cannot multiclass -- focus. Intesne, unbending, unyielding, immutable focus. Ritual. Inner order and development. Logic, purposefullness, and reason. In order to focus that tightly, one cannot waver in the slightest -- they must be an unbending rod of internal and external order. For much the same reason as the Bard, but nearly the antithesis. Monks cannot deviate from their practice, or they can never return. They don't draw their power from order itslef (like the Barbarian draws on at least a little chaos for their power), but they draw it from the ritualized and rational honing of their body. Focus, focus, focus.
I could accept a nonlawful monk if they somehow described how they would keep the intense focus and ritualism of the class.
Paladin: Must be Lawful Good. Why? Well, the good should be fairly obvious. The Lawful is implied by their adherence to a code, and, yes, like a monk, their intense unbending focus. The Code goes a long way for this, codifying and regimenting what a Paladin can and cannot do -- anyone subscribing willingly to a code dictating honor and behavior within society's norms is Lawful. And a Paladin has the additional dimension of focus. In addition, they've got a bit of Cleric in them, granted supernatural powers by a force that they can offend.
Can't really envision a paladin who isn't LG and fits the description.
Rangers: Described as mostly chaotic good, which fit s with their vigilante/wild-man aspects quite well. They don't conform well, in general, and they also are generally noble people with a general concern for the happiness of those around them. Exceptions, however, are not uncommon. They could be as ordered and focused as a Monk, and as wicked and depraved as the worst villain.
Rogues: Described as mostly chaotic, because most of them probably are at least sneaky and willing to cut corners and be flexible where others have focus.
Sorcerers: Described as mostly chaotic, because they surrender themselves to their magical powers instead of codifying and researching them. Sorcerers are probably the epitome of a spellcaster who doesn't need much focus...just the natural talent, and some time to practice it.
Wizards: Described as mostly lawful, because the research and (yes) focus required often favor the ordered and disciplined. Even a rebel could cast a spell, though, if he did the work.
Allow me to be the one to say it: LAW GOT THE SHAFT!
All of the above make some sort of sense from the baseline of the game. All of the above are also subject to exceptions to the rule, though rare.
You can blend the powers of a bard with the powers of a paladin and have no real problem with the classes.
However, you can't blend the dedication to flexibility and bendibility a bard has with the dedication to focus and adherence to a Code that basically defines you as a specific being that a Paladin has. Work around either of these (purely flavor) restrictions, and you've got yourself a valid character.
