I personally like the whole double-kick-drum/growled vocals thing, especially the sudden changes to it that Opeth use. But I know what you mean, it's not to everyone's taste and it can definitely get boring after a while.

They're not strictly the same genre as Opeth, but you might want to check out Mastodon (if you haven't already). Particularly the Crack The Skye album, and the songs:

Oblivion http://www.youtube.com/watch?v=s6WGNd8QR-U
and The Czar http://www.youtube.com/watch?v=Jx2fp-kKOIw

To me, at least, they have a very similar feel to a lot of Opeth's albums. Some incredible guitar playing as well.