he said Portal 2 was "the last traditional Single Player game from Valve" and that even had its own co-op mode. if Portal 2 is a "traditional single player game" then what is a non-traditional single player game and how does it compare to co-op and/or multiplayer?
It would be one that mixes both into one.
He said that it would probably be Valve's last traditional SP experience. What that means exactly isn't overly clear, not to mention the statement wasn't definitive -- if Valve can't tie a social element into HL3's campaign in a way that makes sense then I doubt it'd just go ahead with the idea anyway.