Make sure your copy protection does not annoy customers. That’s been mentioned earlier. When I was working at Indiepath, the GEOM game used to have security system that wouldn’t allow users to install more than one copy of the game.
Luckily one of the customers complained about the issue and we had a chance to fix the problem:
I started the process to purchase Geom, then canceled in the middle. The price of the game is a little on the high end, but not too bad considering it seems to be a good game. However, if I can’t even install the game to both my desktop and my laptop for $24.95 then forget it, you can keep it.
This is exactly why negative feedback is so welcome. If he hadn’t said that GEOM would probably still contain the copy protection. Luckily he mentioned it! Today it allows multiple copies to be made for the user. That’s enough to keep most of the pirates away but still allowing honest people to play the game (like in the case they have more than one computer or so).