The most important thing for me is that the lyrics fit the feeling of the album or the song... the right words for the right music and the right band, you know? I like lyrics that refer to Nordic mythology, but that would be ridiculous if the band didn't sound like a bunch of berzerkers thrashing away. Hell, I like some lyrics just because the syllables sound good, even if it's not even in English.
Some of my favourite lyrics:
Bloodbath - Eaten
Borknagar - Colossus, Dawn Of The End, Rivalry Of Phantoms
Arcturus - Alone, Painting My Horror
Emperor - Curse You All Men!, Thorns On My Grave, With Strength I Burn
Zyklon - Hammer Revelation, Chaos Deathcult