I've been running an ACKS campaign for a couple of months now, and if there is one main difference between ACKS and other retroclones (besides the rules for high level / ruler style play) is the inclusion of racial specific classes. Dwarven Vaultguards are very similar to human fighters, but it is a specific class available only to dwarves. Elven Nightblades are a magic-user/thief combo class that takes some abilities from each, but not all (no open locks or find / remove traps, much to my party's dismay).
The thing is, these classes are unique to each race. They are different from the standard classes. It's been an excellent roleplay tool, and it helps folks think of races in the game as other than just a set of adjustments to a class.
Now, I'm not saying you can't have dwarven fighters or elven magic-users in your campaign - I'm sure I will when I run my next campaign of length, which will probably be a slightly houseruled Swords & Wizardry Complete game. But I think there should be options available for those that want to play something that more "racial specific" in the way of class.
It might also be an excellent way to give the halfling a better definition other that "short thief".
Hopefully I'll have something for the morning, and if I do, I'll post it here. Well, not THIS post mind you, but a new post, but here on THIS blog ;)