FvJ is a good movie, and that's because it understands it's place. Everyone involved knew that slasher movies are complete camp, and to try and make one serious is idiocy. Anyone that went to see FvJ either loved it, because they wanted to see a campy horror movie, or they hated it because they can't stand campy movies in the first place.

Underworld, on the other hand, was touted as a fairly serious movie. There weren't any one-liners or appluase-seeking death scenes, and the story was attempting to be serious. However, the character interactions and plot itself had so many holes and rips in them that it was completely unwatchable. They tried to make a serious movie, but cut a lot of corners while doing so, and that's what constitutes a bad movie.

Just so long as moviemakers understand what kind of movie they're making, they'll have a better chance of succeeding. The people responsible for Underworld tried to make a serious movie, but said "eh, it's an action/horror, so we can get away with giant plot holes and shitty character interaction." They were miserably wrong.