Perhaps metal is the easiest/most fun to pick on in this aspect.
Pre/post, progressive, death, core, and sometimes a country of origin (usually a Nordic country). One dictates how low they detune. One dictates how "complex" the rhythms are for the jugga juggas. One dictates how angry/demonic they are. ...and NEVER mix up Sweedish post progressive death core with Viking metal.
Sometimes I miss the days of glam, death, and thrash metal. Sure, there was the new wave of British metal, but it wasn't like you weren't allowed to listen to other metal of the era. Depending on the year, you couldn't be too picky with your metal friends or you wouldn't have any. Perhaps the popularity of metal overall is why guys can go nuts with the sub-genres.