I feel that to truly understand Opeth's progression, you had to have experienced them in their proper order - that is, as they were being released. When Orchid first came out, there was nothing that you could really compare it to. Morningrise was a lot harder to get into (musically it's more dense,etc.) compared to the immediate catchiness of Orchid, but eventually it made sense. MAYH completely shattered my perception of Opeth and I thought it was over for them. Eventually, I saw the light
I think that it must be so very hard for a fan to understand the genius of Opeth's early material if they started with BWP or Deliverance. Say what you will, but us older fans had no other Opeth albums in our subconscious to cloud our judgement. Listening to Opeth backwards would be brutal IMO! I don't thinks would make sense if I heard Still Life before Orchid. Can you imagine a time when some of us were waiting for Opeth's new album after Morningrise and wonder what direction they'd go next?
In any case, even though I've been with Opeth since the very beginning, I'm thinking that BWP may be their most complete album (up to BWP, Morningrise was my favourite).