The lyrics themselves aren't necessarily as important as the "spirit" in which they are written. In other words, the theme is more important than the words themselves. That isn't to say the words are meaningless, but just not as critical to me. This becomes more of an issue in Black and Death Metal as the lyrical content(or "spirit") is, for me, an important part of the whole feel of the music. It isn't the be all and end all, though. For instance I love Immortal, but the lyrics were perhaps, interesting back in 1994-6...by 2003, etc. the Ice & Mountain theme was tedious to say the least - but the music was just magnificent.
Vocal style and delivery is a make or break thing for me. A poor vocalist can kill a band for me. Conversely, an outstanding vocalist can rescue mediocre music from the dustbin of metal history.
One of the primary reasons I despise Metalcore(aside from it's stunning lack of creativity or originality and PC attitude) is the use of whiney, emo-ish vocals combined with that unbearable, mind-numbing shouting and screeching. No matter how heavy the music may be(orginal or not) that vocal style is anathama to me.
