I'm glad that people have brought up Bioshock. I played the first one, enjoyed it, and now I just recently got the second one and am working through it. The length of the game is definitely enough to justify its price (at least if you wait a little while for the first price drop.) They are excellent examples of what can be made if you care enough to write an engaging storyline, unique and immersive art-styles and environments, and the opportunity for lots of exploration beyond what is necessary to simply get to the level's finish line.