Actually MS is not very compatible. MS is only *PARTLY* compatible with itself (i.e. win98 software MIGHT work on XP). Linux can run many windows aps and linux software is far more version-flexible. FreeBSD can run aps for linux, unix, and might have wine for windows aps.
I also doubt Mac has particularly outstanding performance, but I can't give any real evidence as I haven't used a mac for a while.
I also want to point out the "used more so less secure" is a common misconception. Windows was never coded to be very secure. OpenBSD, however, was and it shows.
A real secure OS does not embed a browser deep into the system, and that's just a single example. I also believe linux/BSD being open source makes them less prone to exploitation.
IceFire: Smart companies never use a windows backend, at least, not pure windows. I believe *nix systems still holds its places as the most used server OSs. A different matter for end users though.