So I've made two mac 'n cheese recipes over the past few months (Joyce Goldstien and Alton Brown) and seen a bunch, and found two main variations.
Spice-wise, I've seen nutmeg in some versus a mustard/paprika/onion in the bechamel.
Sometimes it's an egg-based casserole, sometimes just the cheese sauce.
And of course, there is always the question of cheese.
Of the few I've tried, I prefer nutmeg, and am agnostic on the egg.