Half Life 2: Oh look a dimly lit, dark, and dangerous hole that's full of creepy things, why don't you go down there and smack stuff around with your crowbar?
Portal: Use portals to navigate in a-the cake is a lie-testing facility and solve-the cake is a lie-puzzles.
Fallout 3: The world has gone to hell, its time for you to make it a better place by slaughtering abominations, or alternately, looting everything that isn't nailed down and murdering everyone.
Team Fortress 2: Kill everything that wears a different color then you and always trust the guy who keep trying to walk behind you.
Left 2 Dead: Here's you and three buddies and here are some zombies, kill them(the zombies I mean, but we know that you were thinking of something else).
Mario game: Princess peach has been kidnapped by Bowser, go to eight lands and complete the same task at each then fight Bowser to save her.