Not very. It just has to be fun and well done. Killzone 2 is one of my favorite games of all time, but it's hardly innovative at all. The only thing that can be considered innovative is its cover system, but that's just a tool. The game doesn't revolve around the cover system like in Gears. But what Killzone 2 lacks in innovation and story (although I rather liked the story. Not great by any means, but it did a great job of moving the action along and it conveyed, or at least tried to convey, the horrors or war), it makes up for in everything else. Killzone 2 is just plain fun. The animations, AI, graphics, and sound go hand in hand to deliver one of the most satisfying experiences I've ever had. Not to mention the brilliant multiplayer. Instead of giving players every class from the start, it makes players to work their way towards the better classes, thus forcing them to get accustomed to each class individually. That way, everyone has at least some experience with each class. People may like classes that they didn't even think they would.