Windows NT has been multi-user from the start. In fact, due to microsoft's heavy business focus, it's arguable that it even has better multiple account support (a concept separate from multiuser)

Both Mac OS X and Windows NT have pretty weak security protecting local users against each other.

Privilege escalation attacks that require access to the console are usually discounted as unlikely.

This is an unsubstantiated claim.

Certainly, there have been a fair amount, but not for lack of effort on the OS developers' parts.

It can always be better, and cutting edge, modern OS designs are superior, but it's not like the foundations of NT/Darwin are completely rotten.

Um, what?

Lolz, no it doesn't. The securetoken stuff means that directory service stuff is broken without doing stuff like storing plaintext passwords on the device. Windows has had much better multi user support for years.

Apple seems to fundamentally believe that Macs are single user systems, and it's shown in their past few major releases.