Punch them...oh wait. DAMNIT!
Well, if I had to guess, they're probably the type that basically can't accept anything different or new. The type that plays nothing (AND I MEAN NOTHING) but Modern Warfare 2 every damn day reaching 10th prestige, and calling every single person that liked Shadow of the Colossus, Metal Gear Solid, or Portal a "FGT OMG LULZORS". Or something of that nature.
If I'm wrong, then please ignore what's written below:
I usually don't agree with Yahtzee's overall "score" on a game, but he still makes some damn fine points. Hell, I liked Alan Wake, but you have to admit that the monologuing and the use of slo-mo during enemy encounters really hurt the experience.
If your friends are gonna be making fun of you for liking something, then they probably aren't your "friends" and more like acquaintances that hang around you every now and then.
Then again, there's the route that everyone else is saying: Make fun of what they like, so that it's pretty much even. Call Modern Warfare 2's multiplayer unbalanced. Tell them that Halo 3 is merely by the numbers. Announce that Red Dead Redemption is more like a box of toys then a coherent game.
That's just me though. Do what you want.