I am a developer. No windows machine can replace a macbook. In fact, OSX barely makes the cut for me.
My requirements usually include:
* Can I open a terminal that runs bash ?
* Does it have top, ps, ls, tar, man, grep, ssh, rsync, sed, awk, sort, diff, cut, tail, head etc.
* Does it have vim?
* Does it have python?
An OSX system (runs a BSD variant) and is very close to the systems I work on and debug. Maintaining it is a breeze.
OSX is not perfect. The windows management is inferior to, say, KDE (which I prefer). I run KDE on my work machines, and in virtualbox on my mac.
I used to be a Linux only guy. But the reason I switched to OSX, is because all those other apps like email, calendaring, chat, etc that most companies use are only truly supported on mac or windows.
On a windows machine I wouldn't know how to list the files in a folder (actually, I do, its 'dir'. I remember this only because I'm unable to repress those memories completely).