I'll second Spore, Far Cry 2 and CoD6/ModWar2/whatever.
Spore because I was following that for a year before it was released, and seeing the first reveal of all the layers (microscopic to interstellar) blew my friggen mind. And then getting the game and it was this childish p.o.s. where I had already gotten up to the space level within half an hour. WTF? So much promise...
And Far Cry 2 for the creating a beautifully open game world that I absolutely love, then stopping me from free roaming after I have completed the story. The bad ending, the forced missions near the end, the lack of AI (it was like playing Doom, stand and deliver), the Americanisation of Africa (Jeep wranglers? wtf? Can you say Hilux?), the insane amount of travelling to and fro and running into respawned enemies ruined it. That and a complete lack of SDK, so the fans couldn't even fix it.
CoD6/ModWar2 for completely bollocksing up the best multiplayer ever. Where, the fuck, are the dedicated servers? Thankyou IW for destroying the entire gaming clan concept. Nice maps, nice guns, nice perks, nice everything else. But no dedicated servers? You lazy motherfuckers. I live in Australia. Connecting to the USA means going through Singapore then Asia then the Middle East then Europe then across the freaking Atlantic. Why would I want to do that just to frag some fuckers? Oz has some great clans, an just because you did a half-assed port to PC they are happy to sit around for the next shooter. Did these companies not notice how huge Counter Strike was? If you make an FPS for PC you have to give it dedicated server abilites and SDK, that way your game will live on for YEARS. People are too consumed with making a quick buck out of 13 year-old Christmas presents and casual gamers, the Age of Hardcore Gaming has come to an end.