What are games that you think had great potential, but didn't turn out as good as they could have?

The Sonic series in the 7th gen is my answer:

Sonic 06 is self-explanatory

Sonic & The Secret Rings had an interesting concept, but threw the potential away by pretty much not letting Sonic stop running

Sonic Unleashed didn't have much platforming in Daytime stages, and the majority of the game was Night stages.

Sonic & The Black Night had the same problem as Secret Rings

Sonic Colors was half-assed in terms of game length

Sonic Generations was also short, and should have had a better controlling Classic Sonic and story.

Twilight Princess and Brawl
Tp could have not have wolves
and Brawl just looks better at e3 then the final game

pokemon colloseum and xd if they would have made that like the games oh boy the gamecube would have been passed n64 sales O_O

Every Final Fantasy game since X...

Prince of Persia reboot.  The platforming was good, artstyle was good, but combat was awful.  

The Last Guardian cause it wasn't released

FFXIII cause it had the power and resources of the 7th gen yet it fell short in almost every front, specially with LR

Playstation All Stars

The Wonderful 101 needed a multiplayer versus mode with online. That would have kickstarted some more interest in the finer points of the game's mechanics -- which are sadly misunderstood and even miscommunicated in the game's tutorials. You have to find PG's youtube videos to describe the gameplay mechanics and strategy in greater detail to really get how to play and enjoy the game.

Nintendogs if it was a detailed breeding simulator I would be much more interested.

But yeah sonic games could have been better, Sonic fans are dropping off the earth and now I think I'm the only normal one left.

But I thought you liked Sonic Unleashed, Nintentacle!  Anyway, Mario Kart Wii had a lot of potential thanks to its online component, but everything went downhill pretty fast..