NIS and LDAP both do pretty much the same thing as far as authentication goes. The biggest problem with AD is that it only talks to other microsoft products, and not even all that well with anything but the latest versions of that. People end up completely tied to the microsoft way of life. I don't live on an island, but I find that I have extraordinarily little need for anything microsoft makes.
Thanks. I'm on my way out the door but I'll have to look into LDAP and NIS. My school is big on AD so that's pretty much all I know.