Ack-Ack you have to make the game playable WITHOUT the optional USB joysticks and USB keyboards to get it approved for that platform, if I recall. You have to meet their standards or they won't carry your game. The rare exception seems to be the "rockband"-style games -- no doubt making up for it with proprietary hardware that gives the consoles more money in licensing fees anyways.
Further, game companies such as Valve have been making PC-only games and porting them to console for many years now. They have mentioned the steps they have to go through, including all the ones I've mentioned (level redesign to reduce geometry per maps, level of detail decrease, texture size decrease, simplification of control/commands (often including adding auto aim and "helpers" due to the console nature).
It was rather interesting reading their comments when they were porting the HL2 episodes to console, but they're not the only ones.
So yes, straight from the game companies' mouths, I do know a few things about it. I'm not saying I'm an expert. If you're starting from scratch you build in the limitations and you don't have to worry about it. If you've got a fully working game for PC already and need to port it to a console it's an entirely different kettle of fish and takes a lot of work. THAT is what costs money. The man-hours to redo the game to fit on the console.
P.S. I never said you couldn't make it cross-platform compatible on the same servers. Often this is not done because of politics of the consoles, or differences in gaming experience on the different consoles (to make up an example, say there's auto-aim on the XBOX but not the PS3, then putting those players together is unfair).