But much of what you've listed there can be picked up for less.
A good case, motherboard and PSU can be had for ~$100 each; you can pick up the NH-D14 for ~$80 which will run circles around the iMac's cooling; 4x4GB of RAM can be picked up for $140 and Windows 8.1 OEM for $100.
And don't forget you'd need to buy that same copy of Windows 8 for the iMac, otherwise you're stuck with less performance and less titles.
You could cut costs even a bit more without sacrificing any performance, too. All up that would shave off about $400 and in terms of thermals, noise, expandability and upgradability, the PC wins out.
Ultimately I think the iMac is great value, and it offers a form factor which you can't ever build. But if you're buying a rig just for gaming and form factor doesn't matter to you, then building your own is probably best.
good point about the windows license for the mac. ok, so cut $200 from the cost of the PC. Add an $80 webcam, maybe a .. thunderbolt controller, unless the cheap motherboard you suggest has that. Does it have a good DAC, are all the USB ports 3.0, etc etc.
Your point about "you can get cheaper [blah]" is a bit silly, no? Sure, I could have probably built some cheaper computer with crap components but kept the cpu/gpu the same, but that's not a good comparison. Obviously the iMac is a "premium" product.
Yes. If form factor doesn't matter, if a quality LCD doesn't matter, if some no-name inefficient power supply is fine by you, if sketchy warranties are no bother, by all means head to your local best buy or fry's electronics or newegg and build something. I recommend everyone build their own computer at least once or twice, it's fun. Hell, in late 2010 the last time I thought about buying an iMac I did just that, and I dare say it was kinda pretty inside. It ended up costing more than the iMac I would have bought (around $2500), I regretted the decision pretty much immediately, and ended up buying a mid-2011 11" macbook air so that I could keep using Mac OS heh.