For me, it's FUNCTIONALITY and STABILITY. I tend toward Windows XP, Windows 2003, and Windows 2000 for that reason.
I know people are going to give me crap about using Windows and saying "stability." But guess what - there are specific applications I want (among them, WordPerfect 2000, FrontPage, Remote Admin, Pinnacle Studio, Quicken, and various games); Linux isn't an option.
I'd consider VMware - assuming that I hadn't already set up my main Windows workstation already. And I'm considering a high-end server and VMware for my network infrastructure, and shutting off several of the boxes I have on currently.
That said, I do have a Linux box as part of my home network. I'm using CentOS as a secondary front-end mail server. (I have a service - NO-IP.com - as the primary front-end.) I also use vsftpd for my FTP service (which is normally not active on my site).
I'm sure people are asking "So why IIS? Why Exchange?" Simple - because that's what I'm trained in, and that's what I can support easily. Switching to Apache for web services means a learning curve that I don't want to deal with. Ditto for setting up all the component pieces of my web site - the blogs, the BBS, etc. I don't use many of the components, but they ARE there, and they WILL work.
I could care less whether this stuff runs at a command line or in a GUI. In fact, many of the operations I do in Windows that most people use the GUI for, I simply drop to command line. I don't care what the interface looks like; I just want it easily understood for me when I have to maintain my systems. If it isn't, it'll get replaced.