These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Covered Up: While not exactly the case, Mikael tends to name songs and albums after obscure progressive rock and metal bands of the seventies. Who honestly remembers bands named Blackwater Park, Still Life, Master's Apprentices, or Heir Apparent?
Though the original is still far more well known, there's more than one person who is unaware that "Would?" was originally by Alice in Chains.
While it isn't a cover, anyone who has listened to Camel's "Never Let Go", can tell that "Benighted" uses many elements from that song. This isn't surprising, as Mikael cites Camel as a huge musical influence on him.
Crowning Music of Awesome: While many fans dispute on which songs deserve the title, it is unanimously agreed by all fans that the epic title track of the epic Blackwater Park album deserves it. "Bleak", from the same album, is also considered an example by most fans, as is "The Drapery Falls".
"Black Rose Immortal" off of Morningrise is a fan fave, most likely because you must be a true fan to make it through the song's ridiculous length.
Of course, there's also those who take Ghost Reveries, Still Life, or My Arms, Your Hearse as their most epic album (when it's not Blackwater Park). Hence songs like "Demon of the Fall" or "Ghost of Perdition" get a lot of love too.
Epileptic Trees: Many speculate that Mikael Akerfeldt is one of the nameless ghouls in the Swedish hard rock band Ghost (known in North America as Ghost B.C.). Some feel that he downright gave it away with the band's cover of "I'm a Marionette" by ABBA, as he's a huge fan of them (and has been known to wear their shirts during Opeth concerts).
Fan Dumb: Has come out in full force after the release of Heritage and still had not let up by 2014. Many fans spend a lot of time whining about how Heritage wasn't a "real" Opeth album and that the band name should be changed. It has become worse with the release of Pale Communion, which more fans are complaining is 'worse' than Heritage (despite the fact it's more of an improvement for many).
Magnum Opus: Generally considered to be Blackwater Park, to the point where the band has been known to perform the entire album live.
Mondegreen: There are lots of them, possibly due to the constant use of the Metal Scream. "Deadly badgers made my wreath" (and variations, such as "Deadly badgers rape my dreams") in "The Drapery Falls", "Fresh made tacos of the earth" in "The Baying of the Hounds", etc.