Well I haven't used Mac but I don't think there's a central repository where you can access all or atleast most of the softwares? Like apt, yum, smart, ports in Linux and BSD's. Yeah they evolved out of the need for a package manager but still a central place for all the softwares you need is really good IMO.
Then for many people, it is really essential to know the source code of the software you use, that is the case in any open source OS but not in OS X.
Then compiling a kernel has really nice advantage performance wise. It was previously possible with a bit of help from Darwin project but it's not so possible now
AFAIK. But, well these things are required by a small minority of people not a huge majority.
Then, it's customisation of the overall OS, now that's possible in linux distros like gentoo, LFS and all. But, no proprietory OS allow that.
And anyways there are a lot of open source softwares used by apple like cups for printing, konqueror engine for safari, apache is the inbuilt fileserver in Mac OS X. Both OS'es have different users with different needs. And maybe andy is the person who can be more helpful here cos
AFAIK he's the only person who's used both OS'es extensively at home and all 3 if you count windows. He still needs to flirt around with windows everyday
