Being an avid Planescape player from the early-mid 90's, I thought that campaign setting had a robust take on what alignments meant... after all, the very substance of the Outer Planes was made up of the belief/consciousness of the Multiverse's denizens. A couple of things to consider: Order/Chaos wasn't much of a moral axis than an ontological one.
Lawfulness reflected one's belief of reality being externally/objectively based/defined/grounded; one's viewpoints, perspectives, understandings derived from truths outside of oneself. [The Lawful Neutral plane of Mechanus expressed such qualities in a number of ways, such as the environmental effect of all spoken languages from any tongue being understood identically by all.]
Chaos indicated that one's belief of reality was primarily (or even solipsistic) self-created, subjective definition/genesis, intuitionally granted, etc... or the existence of an objective universe isn't considered important and/or knowable. [The Chaotic Neutral plane of Limbo was a maelstrom of constantly changing elements, a soup of possibility that only took solidity with one's thought/willpower.]
This strikes me as a very elegant and, philosophically, quite interesting approach. What I like about it (and actually, what I liked about Planescape) is that it is completely, and in fact sensibly, incoherent.
Which is to say, there's something pleasingly postmodern about the notion that there is no Truth, just many approaches to it. You might even call it radically postmodern, in that it doesn't just deny that we can know what the Truth is; it implicitly says: there is no Truth, just many truths; deal with it. But the incoherence isn't just pleasing in that sense - it also makes the game work. It says to the players nothing less than the following: "We, the designers, take no position on religion or morality or philosophy or ethics. We're just giving you a toolkit to have fun with. If this involves killing orcs in dungeons, fine. If it involves actual semi-serious battles of beliefs, that's fine too. Go for it, and remember, we don't care what you think." And that's great.
D&D: postmodernism but with orcs. Of course, I knew this years ago, but I believe in the value of repetition.