Mirror's Edge - The gameplay was both unique and entertaining. If the game didn't lose all sense of flow and immersion every time you failed and instead allowed the action to continue smoothly like it would in a Prince of Persia game, it could have been one of the best games of '08.
Metal Gear Solid 4: Guns of the Patriots - Solid Snake is a believable badass with realistic flaws, understandable emotions and real depth. He's quite easily one of the best-constructed and most interesting characters in the history of video games.
Metroid: Other M - The first-person mode works surprisingly well. The targeting is solid while the dodging mechanic is intuitive and successful.
Call of Duty: Modern Warfare 2 - It is cinematic and compelling.