This is exciting.
I've read they didn't tell anyone about the touchscreen on the DS until late in the game.
So I'd guess this Revolution secret is some feature or innovation that's fairly substantial.
I would eliminate microphone altho having voice features more integrated into games could be exciting. But I'm not sure how they would do that with the controller setup they have. Voice recognition would seem to require a mic next to your mouth. I just can't picture that being a requirement for every game.
Local wireless play? This isn't it either. AT least this isn't the secret feature. I'm pretty sure this is already assumed because they said the Revolution will be wi-fi. If Cubes can be hooked up together to play Mario Kart

D with wires then this is already in there.
I don't even know where to guess myself.
Analog buttons? Eh I just don't see as being a big secret.
I might guess that some old games could be played with over wi-fi or updated graphics of old games or you'll be able to use the controller in older games. But that doesn't seem like a big feature to keep secret either.
Maybe it will be hi-def after all? lol.