So, what PC gamer "killer apps" exist that call for the purchase of a PC?

[okr]

I have to correct myself: "Killer apps" on established systems are possible. The last game that attracted lots of new people to PC gaming was World of Warcraft.

@vlad: Yes, that's possible (and not only in Korea). Maybe I'm underestimating the impact of SC2. The weeks and months after release will be interesting.