Expansions usually have less content than before, though. If there's similar or (more likely) MORE content than the previous game, a sequel makes more sense.
People are just bitching because it's a quick turnaround for Valve, and because they really like to bitch.
C'mere, Frankie-Baby! Grab a slice o' these BALONEY TITS! - Clarke Snyder, 7/22/2006