I perceive morality in relation to goals.
D&D defines Good as "concern for the life and dignity" of intelligent beings. Assuming that every sane being, regardless of alignment, shares these values, they would have the same two basic goals: A) improve their own quality of life and B) improve everyone's quality of life. A Good act is any act that promotes B at the expense of A, and an Evil act is any act that promotes A at the expense of B; acts that promote both A and B are Neutral acts, regardless of which goal motivated them. (In the absence of some other moral value, harming both A and B at the same time is insane; this is a big part of why I hate BoED so goddamned much.)
Thus, the measure of a character's position on the Good-Evil axis is the proportion of the value they assign to the two goals; a character's Goodness is the ratio of B to A and their Evilness is the inverse. Being "pure Good" or "pure Evil" is a form of insanity, in which a character places no value on either self-interest or on the greater good.
The vast majority of acts performed by any sane character will be Neutral acts, and most sane characters of any alignment will perform Good and Evil acts when the benefit greatly outweighs the harm. Paladins are both Good and prohibited from Evil acts, but they're not obliged to choose the most Good act, or even to choose a Good act over a Neutral one; the same applies, in reverse, with Blackguards. Sometimes a Neutral act is more beneficial than a Good act, and even pure Good will choose it; likewise sometimes even pure Evil will choose self interest over inflicting harm to others.
You can try to enforce an alignment system where Evil is direct opposition to the greater good-- but then you would have a system in which Evil was almost completely non-existent, limited to demons and psychopaths. Under this system, Lawful Evil as represented by the Baatezu wouldn't be Evil, nor would the majority of historical or fictional villains.
You can apply the same logic with the Law-Chaos axis, except based on different values. Every sane person values both the order of a code of conduct-- whether that's a formal code of conduct or simply a personal code-- and the freedom to do whatever they want. Following your code of conduct at the expense of your freedom is a Lawful act, and doing what you want (or what you have to) at the expense of your code is Chaotic. Following your code when it's what you want to to do anyway is, of course, a Neutral act.
Of course, the more a character values living by a code-- the more Lawful they are-- the more extensive their Code of Conduct will be; most formal codes of conduct will have many more provisions than any person can reasonably observe, that more Lawful characters will observe more completely.
Pure Law and pure Chaos are completely alien to us because it isn't possible to define "order" or "freedom" in the absence of other values. That's why there's no Law/Chaos equivalent to Paladins and Blackguards and why it's almost impossible to base a campaign around Law vs. Chaos without incorporating the moral dimension to display the contrast.
You could attempt to define the Law-Chaos axis differently, but I haven't seen a satisfactory attempt yet-- any definition based on interaction with tradition or authority or the law is too culturally subjective to be useful, while a definition based on the existence or absence of a personal code of conduct would have the same problem as defining Evil as active opposition to the greater good. Definitions of "the good of the many" versus "the good of the few" are meaningless and inextricably bound with definitions of Good and Evil.
This is where the alignment restrictions on classes come in. Aside from the Paladin, who arguably gains his powers from his Code of Conduct as much as his deity, I don't think there's a single class whose alignment restrictions make a lick of sense.
Traditionally, Paladins are Lawful and Good: like any sane character, they value their freedom and their own self interest, but they value their honor and the greater good more. They are prohibited from committing Evil acts, but they are otherwise free to choose whether Law or Good is more important to them; a Paladin can legitimately choose "doing the honorable thing" over the greater good, or vice versa, and can even choose self-interest over either Law or Good, as long as they still show a strong preference for Law and Good and never cross the line into Evil.
I think this is why people argue over Paladins so much; their concepts of moral alignment and ethical alignment are so intermingled that they can't separate them-- which is why they can't tell the difference between smiting an unarmed demon-cultist and smiting an unarmed pickpocket. They're both "wrong" by the terms of liberal jurisprudence, but-- assuming the Paladin's code is based on liberal jurisprudence-- the former is a Chaotic act (and a Good act if it hurts the Paladin) while the latter is an insane act.
Of course, if dispensing field justice isn't against the Paladin's Code, they're not even Chaotic acts; smiting the pickpocket is only Evil if you assume that his death causes more harm than his thefts do, and smiting the demon-cultist isn't an Evil act under any circumstances.
I have no problem with Chaotic or even Evil Paladins, as servants of Chaotic or Evil deities, but I am opposed to Blackguards because the proper antithesis of pure Good is not pure Evil as I've defined it; the opposite of pure Good is being opposed to the greater good and being willing to sacrifice yourself in order to hurt the greater good. This is only appropriate for characters with demonic patrons, and such characters are always going to be short-lived.
Assassins are assumed to be Evil because they are conceived as killers-for-hire who don't care about their target as long as they get paid-- they're willing to compromise the greater good for their own benefit. This isn't really a necessary assumption, since an Assassin isn't necessarily motivated by profit-- an Assassin can easily be altruistic, killing for the greater good, just as an Assassin can have a strong personal ethos that guides their actions.
A Lawful Good Assassin, as rare as he is, is still a cold-blooded murderer, surgically removing threats to order and the greater good. He doesn't have honor, but he has his integrity-- he'll never betray his liege for money, never kill anyone without just cause, and he'll carry his liege's secrets to the grave.
A Chaotic Good Assassin is even rarer. He's got a simple code-- "no women, no kids"-- and he might make an exception if the mark is nasty enough, but he never kills anyone who doesn't have it coming. He kills tyrants and corrupt authorities so that regular people can breathe a little freer.
It makes sense that Warlocks would tend toward either Chaotic or Evil-- Evil has no problems keeping their promises to fiends, while Chaotic has no problem breaking them-- but the existence of Star and Fey Pact Warlocks says that Lawful and Good are perfectly viable choices. (Why no Celestial pact, by the way? "If you promise to promote the greater good, I'll give you the power to do so" is the same deal Good Clerics make with Good deities.) There's really no good reason not to allow Warlocks to be any alignment.
Monks and Barbarians are a personal pet peeve of mine, but they're actually the most reasonable alignment restrictions out of all the ones I'm opposed to-- which is pretty much all of them, except Clerics and the aforementioned Paladins. Barbarians are wild and undisciplined, following their instincts over formal rules and laws; Lawful savages are probably Rangers. Their opposite number, Monks, gain their powers from discipline-- they don't have a restrictive code of conduct, but they do derive their power from spending hours in meditation and pursuing grueling exercises to improve themselves.
That said, I'm still opposed to the alignment restrictions on both classes. Barbarians are savage and undisciplined, but many "savage" cultures are known for their powerful sense of honor. (That's a Code of Conduct.) Monks are disciplined in training, but nothing stops them from following consequentialist ethics or even from acting on every passing fancy.
And then there's the classes where the alignment restrictions don't make any sense at all: Bard and Druid.
Bards can't be Lawful because they wander (like Monks and... everyone else), because they're intuitive (like Sorcerers), and because they follow "whim" rather than tradition-- which is in no fashion supported by the rules and certainly isn't based on the historical role of bards, or skalds, or geisha, or any other kind of "bardic" character.
And Druids are just ridiculous. Maybe, in the AD&D days, when they could only be Neutral-- completely unaligned-- it made sense, but "any Neutral" doesn't make any sense. If a class can be Lawful or Good, there's absolutely no reason that they cannot be Lawful and Good; the Lawful component of their alignment is completely unrelated to the Good component, and Lawful Good is not any more "extreme" than Lawful Neutral or Neutral Good. There is no component of any alignment that violates the Druidic ethos of protecting the natural world-- though extreme forms of any alignment could.
So as much as I like keeping alignment as a tool, I want to see it completely removed as a mechanical restriction on classes.
edit: And then there's the issue of my alignment... which I usually represent as either LN or LE because of how D&D defines "Good"; I'm passionately altruistic, but my definition of "Good" and "the greater good" is different from D&D's and I'm willing to do some vicious, ugly stuff to promote it.