The D&D alignment system is perfect.

Yes. That's right ladies and gentlemen. The D&D alignment system is the best system out there, and it does its job perfectly. It applies to every type of sentient being, and every character, without exception. It has no flaws whatsoever. I haven't been drinking.

"Shjust gimme the keysh I can totally drive.. "

Now I know what you're thinking. Well... no I don't, but judging by the millions of alignment debates that are scattered throughout the interwebs (including a couple I've participated in), many people don't agree with me.
There are many complaints, but generally speaking, there are two.

Argument 1: "My character is complex, not simply an alignment."
Of all of them, this argument makes the least sense to me, because it actually perfectly demonstrates the opposing viewpoint. It's correct. Alignment is not  the same as character. It's not the same as personality.

Counterargument: It's not meant to be. 
Your strength score doesn't tell you whether that strength is in your legs, arms or core. It doesn't tell you how fit you are. It doesn't give you your body type. It doesn't tell you your BMI. It doesn't tell you your childhood or at what age you stopped growing. It doesn't tell you why you are as strong as you are. It doesn't tell you which gym you go to.

Strength, like alignment, is there to perform a function in the game. You have to come up with the rest on your own.

Additionally, yes, everyone fits on the axis somewhere. Are you good or evil? If it genuinely doesn't apply, you're neutral. I know there's more to your character, and yes, you can sometimes act outside of your alignment and still be in character (because character and alignment aren't the same), but you didn't hem and haw for a week about your 13 wisdom not fully representing your dreams, aspirations and life experience. I'll never look at your alignment and know if you're a cat person or a dog person.

Though I could guess..... 

It's one facet of your character. The rest might not even be on your character sheet. You might take some inspiration from it, just like you would any other stat. (I have the highest Wisdom and Intelligence in the party... why aren't I leading it? Maybe my character fears responsibility. Maybe he underestimates himself. Why am I a fighter and not a Wizard? Maybe I idolized a warrior when I was young. Etc.)

There have been attempts to fix it as a result of this complaint of "I'm more complex than that". It's fascinating stuff. The most interesting attempt at a fix is the expansions. Trying to add in another axis (making it 3x3x3). Turning it into 4x4, or 5x5 alignments instead of 3x3. Trying to force it to distinctly cover each personality type, and failing, rather than letting it be a general consolidation.

"Pfft! All humanity represented by 9 alignments?! I'll try 12."

This brings me to the next point.

Argument 2: "It doesn't work."
This one is the key I think. I'm not sure exactly why people don't think it works, yet going through the vast majority of arguments on the topic, one thing seems clear. It doesn't work. It doesn't do what it is supposed to do. It doesn't perform its function.

That function is (according to them) building, and defining your character.

And there's the real problem.
Most of the complaining, comes from people that are using it for the wrong jobs, then complaining that it doesn't do the job very well. "This teacup is a rubbish wallet!"

"Put tea in it? Then my money will get soggy!"

Counterargument: Its job is mechanical. Not creative.

The alignment system is there, so that when the Cleric casts "Detect Evil", or the Paladin uses his ability to "Smite Evil", the DM can look at the thing on the sheet, and generalize, whether or not it works, without a Doctorate in Psychology, and a peer reviewed paper on the matter at hand.

It can move. Like strength drain, or constitution loss, or getting more intelligence from a Wish spell.

It's not there to be a placeholder where your character's personality should be. It's not there to replace the need for motivation. It's not there to replace morality. It is, like every stat, on every character sheet, in every system, a generalization of one element of your character used in game mechanics.

"Can you lift the rock? What's your strength?"
"Do you see the thief? What's your spot skill?"
"Did you grab the crossbow first? What's your initiative?"
"Will you solve the puzzle? What's your intelligence?"
"Does the guardian let you pass? What's your alignment?"

It's not very good at writing your character's backstory for you. Defining your character. Building your character. It answers "yes/no/not applicable" to two questions. Are you good? Are you lawful?


Overall, it leads to some convoluted arguments about what morality is, and there is a good argument to be made that in some D&D definitions of Good and Evil, they are not adequately represented as opposites. This is in part because D&D doesn't go out of its way to define it. Leaving some leeway for DM discretion.

It simply lists some examples of things a good person might do, and things an evil person might do.

It doesn't overcomplicate. It does everything it needs to do to function in game, and honestly, doing more would actually start to intrude on character (No! You're not a good character due to section C of definition 3!). I'm never, ever, going to use it to build my character's backstory/mind/personality/morality. I'm not going to use it as a sole definition of my character's actions any more than I allow my initiative score to.

In every way it needs to be, and really, there's only one way it does need to be, it's perfect.

Although, whenever I try to put my money in it, being a conceptual paradigm rather than strips of leather really holds it back.