Good idea: Bad game...
Too Human. How is it possible to make a game about Norse gods mixed with cyber-punk and it be bad? Too Human, that's how.
Also, Mirror's Edge.
Hey, I know, let's make a platformer where you realy can't see the platforms as you approach them. Or better yet, let's make a first person shooter where you arn't supposed to shoot. Somebody was on some serious crack that day.
Bad Idea, good game...
Dead Space. Not that Dead space was a bad idea, but it was risky putting so much (comic and movie) into a new IP, and it worked. Another bad idea was having RE4 controls for the game, but wait, they improved upon the controls and made them better *stares manacingly at RE5*. So not realy bad idea so much as risky idea (like Mirror's Edge), but it paid off. Dead Space is fraking awesome.