fear factory is thrash/death...rammstein is thrash(simplistic though it is),nightwish is the same old school heavy metal iron maiden was doing years ago...opeth is sometimes death,sometimes rock,sometimes acoustic...just because a band puts several styles in their repoirtiure(sp?) does not mean you need to reclassify them.just classify each song if you must.
those sub genres are for nerds....metalheads just want to listen to metal.i have my car full of everything from sadus to slayer to bloodbath,meshuggah,lamb of god,deicide,exodus,opeth,kreator,soulfly,sepultua,six feet under,the haunted,nuclear assault,skrew,testament,byzantine,suicidal tendances,ministry ,white zombie,prong,megadth...it'sall justmetal.
discussing subgenres is like debating picard vs capt. kirk....it's for da noids