OK, lets look at the current generation (well... current because the WiiU is out) and previous 3 generations.
Current:
WiiU: Weaker hardware than its competitors, but only slightly cheaper than the announced launch prices for the other two consoles. Isn't doing so well at the moment. Third-party developers are hesitant to support it.
Xbox One: May flop due to marketing mistakes by Microsoft and their insistence that the Kinect is mandatory.
PS4. Current predicted winner of the next console generation thanks to mistakes MS made with the Xbox One and the tepid reception of the WiiU.
Previous:
Wii: Weaker hardware than its competitors, but a low price to match. Managed to pull third-party developers back in. Outsold everyone else.
Xbox 360: Did pretty decently throughout, but got burned a bit by the Red Ring Of Death issue.
PS3: Flopped at launch due to high prices (cost more than two times the Wii's price) and new unusual programming architecture. Managed to catch up to the Xbox 360 in the last few years of this generation.
2000-2005 gen:
Xbox: New vendor. Didn't do that great. Only killer end-user feature over the PS2 was Xbox Live.
PS2: Harder to program for than the PS1, but still managed to keep massive third-party support. Outsold everyone else.
GameCube: Decent hardware but lacked third party support due to previous generation. The loss of Rare and Silicon Knights during this time frame only exacerbated this problem.
Dreamcast: Didn't do so hot, Sega left the market about halfway through this generation.
1995-2000 gen:
Saturn: Biggest flop of the generation. In part because of Sega's braindead strategy of releasing the 32x, a competing "32-bit" addon for the Genesis. Also cost $100 more than the PS1 and launched with no warning.
PS1: Easiest console to develop for. Launched $100 cheaper than the Saturn. Outsold everyone else.
N64: Sticking with cartridges lost Nintendo the majority of its third-party support.. This came back to bite them in the following generation.
I'm not going to address Nintendo's handhelds simply because they've dominated the market since their introduction in 1989 and have essentially kept the company running even when it was doing poorly.