Opeth are "Progressive" and that stuff...

It's generally a good idea to start albums off with a bang, so listeners will be grabbed by the first track and then hopefully listen through the rest.

It's the same reason bands always start their live shows with one of their most energetic songs.
 
Prologue.... First 2:30 of The Moor...

People listen to Opeth to hear heavy music (and the beautiful soft parts of course, but all us fans want to hear the heavy) ... so they give us heavy music.. I'd rather play the cd for the first time and get kicked in the face then relaxed