Hardest to record? No way....
Metal is one of the easiest styles to record because there are only two volume levels to it. The one that is right before anyone starts playing and LOUD!

Really.
Metal music has little dynamics at all which is a recording engineers dream! All's I have to do is get a tight punchy sound on everything and the rest takes care of itself.
I agree with John on this though. To make it sound good, the players need to achieve a very high level of musicianship to pull off the parts, especially the drummer. I have hear far too many metal CD's and demo's by unsigned bands that sound horrible because the drummer cannot hit consistent.
John is also right in suggesting some things to think about in getting the tones on the instruments. Tight and bright drums (except that kick drum which should have that nice scooped out midrange sound. You know, where it is all 4KHz and 80Hz! No mids at all!

). Tight and bright guitars. Who cares about the bass because you wouldn't know it is there unless he stopped playing really.....

Distortion on the bass is cool for metal! But the recommendation of turning down the gain on the guitars is excellent advice, but if you are working with unsigned artists, you are going to have a hard time convincing them that a lower gain sound will work better. No, the bad metal guitar players will want to have their Boss distortion pedal feeding a
ADA MP1, with
a BBE Sonic Maximizer and
a Quadraverb with reverb and chorus and eq applied to get their "sound".....

It never fails....Then they wonder why they can't hear their guitars in the mix!
Vocals need just a hair of some tube saturation distortion. That nice sizzly tube saturation distortion!
Compress the hell out of everything and only use a reverb on the snare, and a delay on the voice were it seems appropriate!
Well written and performed metal is a snap to record and mix. It is the badly written and performed stuff that takes considerable skill to even make a clear mix out of. You would not believe some of the things I have had to do with drum tracks from unskilled metal drummers to make all their hits sound about the same volume! I may give the secret away some day, but not now....
As with most styles of music, the signed artists are the ones who got their sound together!
Ed
[Edited by sonusman on 10-19-2000 at 21:38]