It was one of the new iMacs.
First, the sound was way different. It was much deeper in pitch, and may or may not have been a little bit slower.
Performace was erratic. It had a tolerable framerate with most effects on in one map, and on the next it dropped tremendously, and turning stuff off didn't help much at all.
The mouse, though, really bugged me. Those squeeze buttons on the side of the mouse register as MOUSE1. Clicking the scroll ball is considered MOUSE3, so I switched it to secondary fire. It's in a poor place, which makes gameplay rather difficult.
It also had a bad tendency to blur when I looked around rapidly.