I can say that what will incite me to buy are nice odds and sods in collector's edition type packages that you won't get by simply pirating the game, such as maps, art-books and assorted other odds and ends. I think it's important for these things to be some sort of tangible goods rather than a digital add-on as there's always the temptation to pull an EA and make the "added extras" content that were actually cut from the game, which tends to be a bit of a PR gaffe when discovered.
Precisely!
Reward paying customers with a nice book of lore that they can read, a cloth map or a little figurine. Don't punish them by putting malware in your game that destroys their computer! It's not exactly rocket science is it?
