For me, it's Brink. The hype surrounding this game, from the mechanics to the story itself, was hyped beyond belief. Websites hosting podcasts, several hype campaigns, and even a long ass video dedicated to the games lore/background story.
Flopped like a fish out the gate.
The fun game mechanics that were hyped..were dialed back significantly for launch. The main campaign, which was also hyped, was relegated to a mission to mission basis, with hardly any coherency/continuity that tied them together. Then there was the on-line...oh man! This glitch-fest put the nail in the coffin.
Although there were absolutely no memorable characters in the game (including ARK, if you want to consider that as the main character to a degree), lore-wise, the potential was there. Unfortunately, the game came out less than half-baked.
Honorable mention goes to Two-Worlds, you know, the so-called Elder Scrolls killer. What a dud that turned out to be. Although there was some funny dialogue like: "Farsooth, I seek the taint. Pray," Or something to that. Lol
EDIT: One more honorable mention, since this thread seems more retro than anything, I will throw Secret of the Stars for SNES. When hype through a magazine was done only through pictures and someone's worded opinion, this game was being compared to the likes of Final Fantasy IV and Secret of Mana!! I begged my parents to rent if for me, instead my dad buys it (big RPG fan who read the magazines as well and fell for the BS). What a smelly turd this game was. I blame this thread for recalling that game into the forefront of my mind!