Sorry, I don't want to defend what they did with breaking compatibility with any product other than their own (IE, AD, Office, IIS killed Corel, Netscape... and so many other companies). I just don't see AD as a terrible standalone product. But you're right, I don't have a deep understanding of its shortcomings vs, say, OpenLDAP (though I'd love to learn something!).
People said that of IE6. Then came Firefox and despite it running circles around IE it took years before we could feel any displacement. The problem is there's simply no contender to AD anymore, and that's not by virtue of its quality.
MS broke or eliminated other authentication modes from Windows, so companies were forced to use AD (with the associated DNS and other infrastructure).
12 years after introduction, there are no stable/viable alternatives. Competition really was exterminated in this case.