Metal shirts are essential. You can wear jeans or khakis; boots or sneakers; bullet belts or no belt; long hair or no hair. Without a band shirt (or a least a blank black shirt) at a show, you're nothing. That's the only real required part. The rest can be changed around as seen fit. I would rather wear $15 black slacks from Target than blue jeans. They're more comfortable anyway.
As for those who wear Abercrombie and expect to be taken seriously by the metal community: read the LotFP essay titled Scum. In essence, metal is about individuality above all else. If you fucking love Bathory, wear a Bathory shirt, and who gives a damn who else is wearing one. Wear shirts for bands you like because you like them. If you want to wear Abercrombie, you're setting yourself up to be perceived as "one of them". There's nothing about that outfit that will lead people to believe that you have taste or opinions that matter.
If "metalhead" is primarily the way you see yourself and wish to present yourself to others, then wear band shirts as much as possible. Otherwise, you look like a poser and you probably are one. Metal shouldn't be something you hide from the rest of the world so you still look cool at frat parties. Trendy clothing says you still give a fuck about what people think about you, which as a metalhead is something you should not care the slightest about.