Honestly I think there's two kinds of shredding, the really awesome intense kind that's wild and all over the place that you get with Morbid Angel or Slayer, and then the robotic, "hey man look how many notes per second I can do" kind that you get with Dream Theater and Symphony X. TBH none of Opeth's stuff is quite the same as Fred still puts some tasteful parts in his robotic shreds (i.e. the awesome palm muted parts) and Mike's/Peter's solo in White Cluster which was the only one even close to shredding, was just nothing on Trey Azagthoth or Kerry King.
I don't think most shredding sounds dreadful really (though the light speed shit w/ Yngwie or MAB is well...yea dreadful is the word), I just respected Opeth for having a different style that was more melodic and feeling and for producing some of my favorite guitar solos of all time (I.E. The Funeral Portrait's two solos are right up there with Shine On You Crazy Diamond and the gang for me).
Guess I just thought they were "above" shredding and generic metal riffs.