You can make a really easy list of good 360 exclusives not on the PC.
It goes Halo 3, Gears of War 2
Simple. Some others like Fable 2 are in the process of making the trip to PC so I'm going to count them as no longer exclusives.
You can even get achievements in a lot of these game for windows games AND if you so desire play it with the 360 controller. In short if you have a decent PC there is just no reason to get a 360.
I do agree anything on PC should not be considered exclusive. Though that only really matters a ton to the 360, usually any PS3 or Wii exclusives stay PS3/Wii only, its only the 360 that puts all its best games on the PC (both because direct X games are easy to port over and Sony/Nintendo have no interest in seeing the PC thrive as a gaming platform)
Incidentally you can VERY easily put together a PC that performs as well or better then the 360 for the price of the elite (and possibly the premium if you really push it). 400 dollars won't get you an incredible machine, but it will get you something more then passable. Several people have posted sub 500 dollar PC builds that work well enough, and unlike the 360 that 500 dollar PC will be module so you can upgrade the graphics card, Hard drive or whatever for 50-100 bucks when you get the money and prices on current gen stuff drop.