I don't use alignment in my campaign.
<snip>
Have characters (PCs and NPCs) say and do stuff. The audience at the table can get the measure of them from that, just like in any other form of fiction.
For me, what alignment adds to D&D is a sense of the world as a battlefield. Minor conflicts are cast in terms of epic struggles between extraplanar forces, without which, I feel, the transcript of play, especially on the scale of the campaign as a whole, is perhaps less likely to result in an actual story.
On this I tend to agree with Grainger. Of my two active D&D campaigns, the default 4e one has PCs with alignments because that's what the rules called for, but they don't do much work (all but one is Unaligned, and the Good one is a (multi-class) cleric of Moradin which I think, by the rules, would require him to be LG); and if the PCs in the 4e Dark Sun game have alignments I don't remember what they are.
I agree with Hriston about the role of cosmology in default D&D, but I think that can be achieved (and better achieved) without mediation via alignments. All the PCs in my default 4e game are aligned in more or less complex ways to one or more gods or other cosmic entities, and from the lowest levels their struggles have been a playing out of these cosmic conflicts. I just don't find
alignment a very powerful way to do this, for reasons I'm happy to elaborate on if it's interesting.
Because Dark Sun is not a cosmological game in the way default D&D is, I think alignments have even less work to do.
I should add: all the above is about the player-side. On the GM-side I treat alignments as rough behaviour descriptors. And for that purpose the 4e alignment spectrum for monsters/antagonists (Unaligned, Evil, Chaotic Evil) is sufficient.
And unrelated to alignment at all: I take PF, OSRIC and other clones to be obviously D&D - just without the branding. They're no different from the Mayfair Games Role Aids from the 80s.
A different way into D&D is not through the mechanics but through the world conceits - when I GMed Rolemaster campaigns for nearly 20 years using large amounts of D&D setting material and (mechanically adapated) modules I wasn't exactly "playing D&D", but I certainly felt part of the D&D community.