There's almost no comparison.

Open Directory is basically a ldap with MIT kerberos that can sort of integrate with AD. Most folks setup a hybrid (referred to as the "magic triangle") where you use AD for user auth and OD for computer policy stuff. It's not very robust, and Apple isn't exactly committed to the notion of server operating systems, so it doesn't get much love.

You really need to buy a client app like ADmit Mac, or a server-side solution like Centrify to really work with Active Directory.

AD itself is a really brilliant collection of software. Think of it as a fully integrated, smoothly operation bundle of LDAP, Kerberos and DNS -- it "just works".

Coming from a Unix background, AD was amazing to me as there isn't any Unix equivalent at all, except maybe for the ancient NIS and NFS integrations. You get single sign on. Robust Access Controls. The ability to trust the identities of external organizations. A policy engine to do all sorts of management activities at various levels. Oh, and it allows you to easily build fault-tolerant deployments without much forethought.

The reason that AD is so needed is the ease of implementation with EVERYTHING, as long as its MS(ish) based. One AD account can auth a user to any service running on the company systems. For example, I work for a university as an applications dev; My local box login is the same for every VPN and additional university service that we use.

Now, you can do this with OD, but it's not as easy as "built into all windows installations, set it once and forget it".