Are people using a mouse and keyboard going to play against people using a controller? The results will be hilarious (in that anyone using a controller wont stand a chance). Everyone will be forced to use a mouse.

There have been games that have released with this allowance. They achieve balance by offering powerful aim-assist to the gamepad players.

In any other context that would be the foulest cheating... but in the context of game-design for vastly different interfaces, it's a perfectly reasonable handicap.

You wouldn't be able to (and can't) do this on competitive PC/Steambox games though because you can just fake it on PC, so gamepad players would never be able to compete.

It also works reasonably well for cooperative games. I beat Portal 2 with a friend this way.

And I've never been able to play Super Mario Bros. with a keyboard. It goes both ways.

Wait, that doesn't make sense. Super Mario Bros. uses digital inputs, not an analog stick, so it's essentially the same as a keyboard...Are you saying keys are somehow different than controller buttons?

Well, yes. There is a different feel to using a d-pad and using four distinct keys, and it throws me off when I'm playing a 2D platformer. In SMB, the inertia feels totally wrong using the arrow keys or WASD.

I agree, but I think it's because a d-pad does two things: 1) it lets you press two directions at once with a single finger (all four diagonals are actually two presses, e.g. up+left); 2) it stops you from pressing two opposing directions at the same time like you can on a keyboard, e.g. up+down.

Once you get used to these limitations or find a game where diagonal movement isn't required (not sure about SMB), it's really quite tolerable to play a 2D platformer with WASD or arrow keys. But you're still right it doesn't quite have that magical kinesthetic quality that a d-pad does, and I'll always choose a gamepad myself.

Not easy to play on a keyboard using only two thumbs.

Really? I first played it on the console but have no trouble using a keyboard.