When I'm in the mood for listening to the mellowness and subdued Opeth, Damnation hits the spot. Their 1st four albums were similar, at least to me. All classic Opeth - all stamped with the Opeth sound.
Blackwater Park (my favorite), took Opeth to a different level. Mainly, it was there first sonically produced album. They went from raw Opeth to a cleanly produced Opeth metal machine - still Opeth, but more.
Deliverance/Damnation:
Even though they were made at the same time, they are two different albums. I too think they should of been released together. But that being said, Deliverance is Blackwater Park with a tinge of the classic first four albums.
Damnation was the "experiment". They knew this was not meant to appeal to the strict stereotyped metalhead - someone looking to headbang and listen to music that "pounds" you. This was the other side of Opeth, and they warned us it would be like this. I walked into Damnation with an open mind, and have thus far come out with my second favorite song (not just Opeth song, but any song): Closure, and I also love Windowpane. The album shows me that Opeth can be mellow, and that Mikael has the voice to pull it off. I'm not going to compare it to other bands that make a living of putting albums out like this - I will just say that I feel they accomplished what they set out to do, and I like it. I may still magnate to heavier metal, but this gives me another layer of Opeth to enjoy from time to time.