It is attitude more than anything.  Because of where I am in my life now, my velvet is gathering dust.  I no longer dress as a goth, but I still consider myself one.
That can really mess up the posers.  I went to a Gothic shop with my husband to buy him a top hat (like an undertaker type).  In the shop were a lot of young kids (not even college age) in there, dressed to the nines.  My husband and I were in our normal street clothes.  We got stares!  The shop keeper even ignored us, until he heard the conversation we were having about the music playing over the speakers.   Realizing we weren't in the wrong store, he warmed up to us.  The younger kids were still staring at us.  One made a comment about my husband being "too old"  In which my husband said.  "What!?   I'm younger than Peter Murphy!"
You could tell who really was a goth by that comment.  The ones familiar with Bauhaus laughed, the others scratched their heads. 

So yes it is attitude.  Not the clothing or the make up. 

I consider myself Goth, but I don't dress as a stereotypical Goth. I dress usually in color combinations that don't really go together, simply because I like the idea that it hurts other people's eyes just to look at me. Serves them right for staring. 

Would I be correct to describe the substance of being ''Goth'', as a philosophy more than anything else?

I agree, but like I said at this point in my life I have no reason to dig out the outfits.  Actually no time is correct.  I could wear the jewelry at work, but it could be dangerous.   But come to my home.  At the entrance is a copy of the stone from the catacombs  which says "Abandon hope all ye that enter here."  Each room has a bit of goth decor.  So I might have no occasion to dress up, but I still am a goth.   
For that reason I am jealous of my husband.  He has more opportunities to dig out his outfits.   So when we are together we look really strange.  I usually have my work clothes on, and he is wearing a long coat.
But that's ok I get home, relax listen to some good tunes, read a book, and its alright.
Besides on the occasions I get to dig my outfits out, it makes it even more special for me.

The hard thing is for my youngest.  His taste in clothes are very gothic, but he gets called an emo.   I can tell the difference, too bad the kids in his school can't.

as far as being classy while still being goth go's you could just where the more gothic colors. think mrs. adams in my opinion with a little more modernization that is the ideal goth girl look not creepy and evil looking, dark and beauitiful looking. it's still classy but still goth. that's the key i guess.

i dont think it's neccesary to have piercings, that's for each person to decide for themself.