Are you running Windows in a VM?
No.
If not is there a reason you're spending all the money on an iMac just to install Windows on it?
Boot Camp. I want the ability to run OS X when I need to.
If it's the monitor you want you can buy their bad ass monitor.
That's part of it. Monitors by themselves are super expensive, but the whole computer is in that little giant space.
I got that same i7 processor + mobo combo at Fry's Electronics as a packaged sale deal not long ago for ~$300. Basically a free mobo. Buying a 250GB SSD separately would be pretty expensive - not sure how much you paid for the iMac but I'm guessing near $2000. Could probably have save $500-800 building that same machine on your own. (without the expensive monitor included, which is kind of the clincher for iMacs, i guess)
Edit: Ok, after looking at the new iMacs, I'm going to raise my estimated cost of that iMac to closer to $3000,
Yeah, with the education discount it was about $3000. I could have saved that money, but I have had really bad luck with my last 2 desktops that I've built (I've built my last 6 or so desktops). With this, I get peace of mind that it'll work right, plus I get to save all the space (since it's just the screen) AND it's silent. It makes no noise.
Drawbacks were that it didn't include USB3 or the new FireWire version, but I can live with those things being missing.