Solaris and HP-UX are enterprise UNIX, on NeXTSTEP and OS X, UNIX was always seen as a means to embrace and extend with Objective-C frameworks.
By "enterprise" a lot of people understand native Outlook, Powerpoint, Word, Excel, ... Adobe tools, ..., etc. support. On HP-UX and Solaris you get none of these.
As for enterprise desktop UNIX, I bet Red-Hat has more enterprise integration points with IT infrastructure than OS X.
Microsoft web apps suck. The outlook webapp does not even support smart folders, and dozens of other "basic" features that the native apps do support. Same for Word, excel, powerpoint, etc.
This might be an option in the future, but right now, paying microsoft for their webapps is just a waste of money.