Just to add to what's being said here. I always thought that a lot of the racism (despite it being worked on in later episodes - even though it was definitely still there) didn't "show" because of the inherent military nature of the federation.

If he's just taking orders and doing his job, while only making loud complaints of the garbage Cardassian computer (which ALMOST killed Dukat himself in the episode "Civil Defense"), then maybe it doesn't come out much. Like, I had NO idea O'Brien hated Cardassians so much until the TNG episode The Wounded.

I think even the show, Community, does a funny (non-Star Trek related) riff on this type of stuff in the episode "Aerodynamics of Gender." ** By "type of stuff," I mean the idea of being able to just make someone a racist after-the-fact with inserted footage, or dialogue that would otherwise not be part of the character.