I suspect that "harcore" anime fans dislike DBZ, because, at least in America, there's a hipster attitude about anime -- coming from the fact that people who like anime tend to not want to be considered mainstream and their interest verges heavily on the fact that they are part of an obscure and lesser known community. DBZ's mainstream popularity has caused these people to think that it has devalued their anime genre which, before, you had to be in-the-know to be a part of.