The problem is, coh players and some uninformed players are looking at it as just an expansion. The gameplay is very similar, but it stands perfectly well on its own, without coh at all. And likewise, coh stands alone fine also, minus "guild" bases that you get from getting cov also. The "expansion" nearly doubles the content of the game, adds 5 classes to the existing 7, nearly a dozen extra-large zones, bases, new systems for loot, a new gladiator battle system that's damn fun(xp gaining bug or not)
I am hard-pressed to think of an expansion that added this much content short of maybe trials of atlantis for daoc(booo...the expansion that ruined the game) but even that was about half of the content that city of villains has.