Games that badly disappointed?

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Lair: Rogue Squadron but with dragons was an awesome pitch, but the final product was a mess.

Halo 3: For all the buildup about "finishing the fight" the campaign was underwhelming and most of it felt like I was just mopping up stragglers after the action had already happened.

Half-Life 2: Heard for years about how it was one of the best games ever made, found it a plodding and listless bore.