As long as you have fast enough fingers, and can do chord changes well enough, most of Opeth's guitar playing is simple enough, but one that I can definitely think of right now is the sweeping riff right before the solo in Leper Affinity, Im sorry, but for me that is a challenge to play smoothly. Also, some of the more awkward "chaotic" riffs on Still Life are kind of hard to play because most guitarists haven't encountered stuff like that in other music.