well, my primary reason isnt the mix of heavy and soft parts, for a lot of other bands do that as well...most not as well, but thats not the point. I think primarily the creativity, atmosphere and outside the box thinking that they brought to metal is what i enjoy the most. You can argue that many bands have used Opeths elements before, which is true...but no band really sounds like Opeth still...and if they do, i promise its not terribly similar, or quite the same aproach and vibe. Those of you that want to say agalloch, novembre, katatonia etc until youre blue in the face, feel free to do so, but Opeth is a standalone band in most respects....and thats what i like about them as well, the uniqueness. Also, i havent heard another metal band blend heavy metal with classic progressive rock ideals nearly as well as Opeth, which im obviously a sucker for as well.