The black metal scene is kind of over saturated in my opinion and could do with less random kids and more competent musicians.
I agree with hidden tracks, they piss me off.
Really long intros are gay, especially if it's all sound effects and keyboards on an otherwise guitar driven album. There are exceptions though.
I don't mind fantasy / mythology / historical lyrics but it's better if it's immediately apparent what the band is actually singing about. If it sounds like GRRRRR HURRRR GRRRR HUURRRR BLASTBEAT HURRRR but the lyric booklet says something about greek mythology then it's a bit of a waste. That's possibly just to do with me being really bad at working out what vocalists are saying though.
I wish more metal songs had lyrics like those in The Sentinel by Judas Priest. To me those kind of lyrics are much more interesting than half a medical textbook or religious bollocks. In retrospect that makes me sound dumb, but I do specifically mean within the context of heavy metal lyrics. In a normal situation I'd rather read something more serious.
If there's any anti british sentiment in the lyrics, album cover or whatever then I wont buy the album.
To many metal albums feature faeces on the album cover. If you want to put out shit, go to the bathroom, not the recording studio.