This may seem obvious but it is something that has tripped up developers including myself. When designing a game you need to figure out what platform it should be for. The most important part of this is the controller.
The Controller
One of the most popular games on the iPhone is Angry Birds. If that was designed for PS3 it probably wouldn't sell that well. The game is meant to have that tactile feel of pulling back a slingshot and flinging birds through the air. Without that feeling it would be just another physics game.
The same goes for console games trying to make it on mobile devices. The majority of their success comes from their graphics but let's be honest, how many of those are actually fun? They come of as tiny clones of their console siblings and feel like a cheap way to make an extra buck.
So how do you know which controller works with your game? Well, you need to prototype it. If you require swinging motions with your hands maybe the Wii is a good fit. Or if it will require lots of buttons then a traditional console or PC may be best. Is the game simple enough to be controlled by swiping, tilting, and touching a device? Then mobile devices or flash may be your pick.
Graphics
I'd be lying if I said that graphics didn't matter. If you have a team of artists that can crank out tons of awesome 3d models and animations then you might be able to hit the console market. Otherwise I'd stick with the more slimed down markets where 2D and simple 3D graphics can make it.
However, this isn't true in all cases and many times if a game's graphics are quirky enough they can make it on larger platforms with success. To really be able to quantify this you need to look at your target consumer and what would be visually appealing to them.
Marketplace
This really boils down to your players. Who do you see playing this game? If it's anyone 40 and older than you should definitely consider the mobile market or PC games (flash and other). PopCap, for example, has been a huge success in those markets because their games are more targeted to an older age group.
When you have fast paced action with anything more than 2 buttons then a console should be a obvious choice. Especially if there is co-op involved. PC shouldn't be factored out but generally consoles are more accessible to a large range of players. And honestly I'd rather sit on a couch and play then be cramped up in my office chair. :)
Next time you design a game, consider these factors while you are prototyping.