Marius wrote:And the point with a "killer app" is that a single game is so good it's actually worth going out and buying the machine.
Basically. It's that game that EVERYONE buys and loves for the system. If your friends come over and see you playing it, they're in the store the next day buying the console just to play that game, because it made them go "WOW" so much.
Mario 64, Resident Evil 1, Final Fantasy 7, Goldeneye 007, Halo, Metal Gear Solid 1, Mario Kart 64, and GTA 3 are all good examples.
You'll notice that these games may or may not be sequels, but they completely revolutionized the industry when they came out. You never played anything like them before in a single game. They're usually all exclusives to one system too (for a long time at least), and they appeal to a large crowd of gamers (lots of really good RPG's fail to meet this last requirement, and that's why they don't count as killer apps).
There's nothing new out there for 360/PS3/Wii that makes you go "WOW" like that (arguably you can say Gears of War is an exception, but I don't think it's revolutionary enough).