Microsoft was never a web stack company and always did a poor job at it. Their real power is in the Active Directory. It's very hard to find any decent alternative to that (and no, OpenLDAP isn't the same).
The original plans for dotNet were "applications as a service" in 1999 and what is known as their "Azure" cloud service platform is heading the same direction (like the plans about AD in 1994) too.
You're right, OpenLDAP isn't the same. It's more of a toolchain and server to build yourself a directory implementation on top of.
But when we look at our organisation, all we need is simple SSO for a few applications. We don't actually need all of the extensions like GPO, management tooling etc. Incidentally this becomes cost and noise.
