When it came to shattering expectations, I have to say three games did it for me. Prince of Persia: The Sands of Time was one. Somehow I never listened to the people who told me to run and get the game ASAP. But the second I got it on the PS2 and started playing it, I couldn't stop - literally. While the game was a bit frustrating the first time around, once I got the hang of it, I couldn't stop. I still play it regularly.
The next game that shattered my expectations was Half-Life 2. I know much has already been said about this, but after the disappointment that came with the release of Doom 3 and playing it without much of a gameplay angle, I was somehow preparing myself for the worst when I picked up HL2. I went home installed it and the trouble with Steam started right away. I thought "Oh heck! This is gonna be a worse let down than D3 was!" The game finally loaded after an hour, and after staring at an interminable load time for the first level, I started playing through it. I never was convinced of the game's true genius until the "Water Hazard" level. It was then that I truly realized that it was more than what I had expected.
But the most earth-shattering experience for me came from a game that unfortunately didn't get the credit it deserved because it launched along with the biggie - HL2. The game? Tribes: Vengeance. I didnt open the package until I had finished playing HL2 and by the time I got around to playing the second level and the multiplayer online, I had to slap myself to realize how the game had come from nowhere and blew away all my expectations of the game to kingdom come. I have never played a game that was this engaging in single-player, except, perhaps, say Call of Duty. To anyone that hasn't tried out this game, do so - NOW! I guarantee you till love it to death!