Design, durability, and support.
I develop enterprise applications mainly for Windows and I've been using MacBooks for the last two years. I've had a lot of laptops over the years and the design of the MacBooks is top notch, many other Aluminum laptops are really just a thin aluminum shell the MacBook is built by removing material from a solid piece of aluminum, hence it's far more rigid and durable. But the size, performance and elegance is what sets it apart. Even the power supply is better than any I've had from dell or hp. I travel a lot and my company buys Applecare for those that have MacBooks, we can go into an apple store and get issues resolved quickly practically anywhere in the US, though I've never had to use it.
I haven't had any problem working with Windows, I use the Microsoft remote desktop app (the one in the App store not the one that comes with office). I also use Parallels to run Windows virtual machines and many of my client have VPNs that I use to connect to remote Windows machines. Never had any problems. I have office 365 so I get office apps on my Windows VMs and on OS X. I generally use the OS X side for Email, Web and Office, then do my development on either a Windows VM or a remote Windows machine.
I've had a lot of high end machines from Dell and HP over the years, they've all been junk compared to my last two MacBook Pros. I'll never go back, and I certainly want nothing to do with windows 8.