Q6600, 2GB DDR667, Asus 8800GTS 640, Logitech G5.
Of course the difference is only slight, but even slight differences can completely change how something feels. During a recent project, we designed a handlebar for the disabled. Test subjects kept complaining that it felt flimsy and about to break. We then thickened the plastic enclosure by .05mm (which won't make a significant difference at all in durability) and added a little more material inside which had no other function than adding weight, and voilá.
The issues were gone. Suddenly it felt like a sturdy handlebar.
Small things like that can have a much larger impact, even when the test subject is not aware of the changes.
Also, I agree that some people are more perceptive to this than others. I don't say this to make myself, grobut or T2A look pro or anything, but it's obvious everywhere. Practice makes perfect, and as such you'll often hear beginning guitar players use a guitar that is completely out of tune. They won't notice.
Even experienced guitar players will sometimes fail to notice that they are making windows crack.
Considering that I've had this mouse for about 3 years now and play quite a lot, I've grown accustomed to how it handles and feels. In UT3 it does not feel natural.
I can't quite put my finger on what it is, but the 'moving the pointer through a bowl of jelly' description is quite fitting. It's like the crosshair has a certain inertia and that it takes a while to react. But once it's moving, it's hard to stop again.
This coupled with inaccurate netcode and hitscan weapons makes UT3 insta quite unenjoyable for me (see couple of posts back).