LDAP vs Active Directory - Fundamental Differences?

[Tod Thomas <tthomas@chubb.com>]

Help.

I need to come up with a fairly comprehensive list of differences
between MS Active Directory and a standard LDAP directory service; such
as OpenLDAP, for a group that may ultimately decide our directory
services direction for the enterprise.

I've managed just fine with a combination of Netscape's directory server
and OpenLDAP.  I've avoided ADS because I'm a big fan of portability,
and interoperability.  I'm uncomfortable with the idea of shifting gears
and going full steam with ADS.  If anybody could provide their own
experience, or point me to some good document links highlighting the
differences, I would appreciate it.

Also, my understanding is that a Meta Directory service provides an
'umbrella' to manage an enterprise wide, heterogeneous, directory server
environment.  Is this assumption correct?  If so, I would be grateful
for anybody's stories, or recommendations.

Thanks - Tod

BTW:  Sorry for cross-posting.