Killzone 2.
The hype train for it was monstrous, it was built up as the ultimate killer app that would set a new standard for first person shooters.
Then it released, and it was thoroughly mediocre. The devs' attempts to simulate the character's weight and momentum just made it feel like I was controlling an obese drunkard, the admittedly impressive graphics technology was wasted on endless grey and brown industrial areas, the characters and story were horrid, and the gameplay a bland laundry list of first person shooter cliches.
Thankfully it's sequel was excellent and improved just about everything wrong with 2.








