After the Black album came out and after years and years of
relentless touring round the world plus a lengthy break, I think
that Metallica lost the ability to write great thrash riffs. I
think the break was so lengthy that their approach to writing songs
changed and that they may never make an album as good as anything
from Kill 'em all to Black.
I, myself hated Load, Reload and St Anger especially as Lars'
sounds like hes hitting water buckets. I would still go to watch
them play but purely for the old stuff.
Slayer have always remained hardcore to their roots because Kerry
King and Jeff Hanneman are still influenced by the same sound and
style to this day. They have an uncomprimising attitude when it
comes to writing slayer songs and is why the slayer sound has never
really changed or evolved.