I dislike when people attempt to pigeonhole works of fiction. People saying things like it's a comedy and should only be a comedy, anyone who writes it different is just not getting the point. The main reason this bothers me is because the best works of fiction are a mixture of genres. A little comic interlude in a serious story often adds to the story. Similar to mixing genres, sometimes the genre shifts to another one entirely. For example, a fantasy story involving magic could end up actually being a science fiction story involving high technology. Sometimes the changing of genres or styles is good, sometimes it is not.
A few examples of changing genres:
-Buffy the Vampire Slayer. The movie was a comedic parody on vampire movies; the television show was a much more serious drama. The movie was not very good, but the television show was, for the most part, very good. The changing from one genre to another was very beneficial.
-Batman. Batman has run the gauntlet of genres. There were horror stories, drama's, romance between Batman and others, the television show was a comedy, detective stories, fantasy stories and science fiction. Some were good, some were bad.
-Ranma fan fiction. The manga has aspects of many genres but most common were comedy, action, and romance; Takahashi even commented in an interview that she tried to vary the story arcs between those three genres. I've seen people try and pigeonhole the Ranma ½ manga as only being a comedy and that really annoys me. Part of the reason, I dislike that view is because many of the best Ranma fan fics are not comedies. Personally I believe that it is easier to write a Ranma fan fic in any other genre than it is to write it as a comedy. The reason I think this is because the vast majority of the humor in Ranma ½ was graphical in nature. It was primarily slapstick and visual comedic exaggeration which is very difficult to write well.
If I ever get around to writing/publishing a fan fic; it is very unlikely that it would be a comedy, though I would attempt to include some even if the fic in question is meant to be a dark fic in the style of H.P. Lovecraft. Examples: people not demonstrated to be abnormally strong would not send people flying (while some are purely for comedic purposes some were shown, by the plot, to be literal in the manga), the Tendo's during the 10 yen arc after Nabiki's date are exaggeratedly poor (the house would not decay that fast and it is impossible for them to sell the stuff as fast as they did), people cannot fit people into their mouth (like one of Konatsu family members did), etc. The super human abilities, spirits, and magic would likely still be in the fic but even that is likely to be toned down.
Though shifting genres is not always a good thing. An example of a bad change of genres was a Japanese film that I can't recall the name of the movie at the moment but do remember the plot. The film started out as a slapstick comedy (flipping the table, hitting people, etc.) and then about a third of the way through it started showing details and explanations for the behaviors seen that completely changed the genre. It showed that while it looked like slapstick to an outside viewer it wasn't slap stick. The guy was really physically abusing her and showed how she was dealing with it (hospital visit, having to clean up the mess the guy made, etc.); if I remember correctly it did not have a happy ending.