When people say AD they don't mean the LDAP part with centralised user accounts. That's been doable for ages.
When windows admins talk about AD, they are talking about all of the things that you can do with group policy and how those policies apply to different containers in a hierchical or cross cutting way, depending on configuration.
With AD and GPO you can:
-choose who has access to which desktops or servers and at what level in a granular or structured way (web admins have admin on web boxes but not mail servers, etc)
-choose what machines have what software installed and in what way
-set things like storage quotas (mailbox or otherwise) depending on a user's position/job
-delegate a login server and storage cache depending on a user's physical location
-enable and disable OS features (developers get IIS and debugging, people in finance don't)
-configure access to shared mailboxes/other resources
So if Jim moves from finance to web development, you drag and drop is user into another OU and add him to 5-10 groups on the AD server. Next time he logs on his access levels, what software is installed, what mail he has access to, his quotas, etc all change instantly.
This CAN be hacked together with a bunch of scripts, a custom repository, NIS/openLDAP, and some other stuff in Linux, but it's not well documented, well supported, or something you can ask ANY linux admin to do and they will do it in the same way.