I have often wondered this myself. You can buy Megadeth remasters for bargain bin prices here, yet all of Metallica's back catalog still costs the same as a new album and for the most part (pre-MoP) sounds like shit. Which is also the reason why aside from MoP and the black album I do not own any of their CDs right now.
ride the lightning has fastastic production considering the year it was released
what I loved most about early metallica though was the tremendous songwriting. All of their albums had such great variety if you consider a song like "fade to black" compared to "blackened" compared to "sanitarium" compared to "live is to die", they wrote songs that were balls out thrash, and then they wrote beautiful introspective songs like fade to black
not to mention metallica has been a gateway band for literally millions of metalheads, so you can't discredit their contribution and importance in the genre (this is more aimed @ greys)