If you have a finite amount of funding to make a game, and you decide to add a coop option to a game that was originally going to just be SP, then your going to have to make sacrifices somewhere; this is basic mathematics.nuba km said:to you really think someone at valve would go 'you know that awesome horror level idea we have lets not to it instead of removing one of our more samey bits to make a small co-op campaign' and that would be followed by 'yes' because if that's your view of how hey add co-op to games you are properly thinking they removed some great single player puzzles from portal 2 in order to make the co-op puzzles if so I think you'll be in for a pleasant surprise.
No I don't think someone at valve would go "you know that awesome horror level idea we have lets not do it so we can make the game coop", because I don't think anyone at valve is that dumb! And your argument to remove one of the more "samey" bits, only proves that you yourself know that adding coop will always have to detract from the SP game... makeing the experience not as good
As for portal 2, while I think it will be great (And I'm actually looking forward to the coop) the SP game probably would end up being a little longer (Or the game would be released sooner) if there was no coop.
Trust me when I say, your wrong. Otherwise all games would feature coop (And I'd be king of all the verse and wear a shiny hat!)