They are only Death Metal in the aesthetic sense of the word. If you listen to a straight up Death Metal band like Morbid Angel or Deicide, you would instantly realize that Opeth are not on that plateau... despite Mikael having said countless times that the best categorization for their band is 'Death Metal', its not hard to see that Opeth differentiates much from the average 'Death Metal' act.
The only 'Death Metal' aspect I find in their music is mainly
1) The vocals, which as of late have become deeper and definately trademark of the 'Death Metal' genre.
2) Some of the song arrangements and the way they entwine with the vocals.
Lyrically, Opeth is definately based around demising themes... death, withering in every sense of the word, but not in the shallow sense that most 'Death Metal' acts are. They do not solely sensationalize or intensify purely death in their music, although they touch on themes closely related.
The ideology with Opeth is that they're out to make good music... and they've used a number of different influences way, way outside the metal realm to achieve their overall sound, and I think its giving Opeth discredit if you simply and bluntly label them 'Death Metal'.