Fear of the engine blowing, especially with all the threads lately. But really, my car has been very dependable. I have put about 40K on it in the little over two years I have had it, and I have not had any problems that were not a result of me messing around with modding the car. I continue to be struck by how responsive the car is and how much fun it is to drive.
Oh, wait, I have the 2000 boxster, it has to be the plastic rear window, especially with the GT3 seats in the car, it is a bit of a chore to get out, chop the back and then lower myself back in. BTW, loving the GT3 seats, took a 3-hour trip to the mountains with the wife and no complaints....
Ed