I guess most prog/power fans (Symphony X as an example) would not like black metal very much. Vocals is one problem. Being a classic metal fan myself, I used to hate the death vocals (though it's in the past now). The second thing about black metal is that the bands mentioned here (Dimmu Borgir, Emperor) use so-called "neo-classical" sound which sounds pretty cheesy if you ask me. I tried to "get into" Dimmu Borgir's Puritanical Misanthropia CD, but after first couple of songs it really starts to sound like background noise that you kind of tend to ignore. Same with Emperor. Maybe it's just me? I do like Immortal, though, they sound heavier and without cheesy symphonical sound. Don't get me wrong - I do like symphonic metal, if properly done. In fact, I'm a Therion fan (here we go, another black metal band, more or less).
Someone mentioned Opeth. That's a completely different story. I think true prog metal fans will appreciate this band. For those who don't know, they can be described as death/gothic, but they are much more than that. Vocals are mostly done in death voice. But - they are very progressive, and use a lot of interesting and fresh sounding effects, such as acoustic guitar in combination with double-bass drumming. Their songs tend to be long, with time changes, slow acoustic parts, you name it. Definitely give them a try. I would recommend Still Life or Blackwater Park albums for the beginners, they are not as brutal as earlier work, and have some clean vocals on them. Just be open-minded towards death vocals.
-GK
