Actually, I tend to rarely listen to "pure" genre music of any kind... I tend to find it boring, especially when it comes to death metal. Entombed and Morbid Angel never really interested me at all. Although most of my music falls within the big realm of metal, they all seem to fall into this category of "Oh, this is a cross between metal and (insert genre here)." Hence your Opeth, Therion, Nightwish, Amorphis, Pain, Ulver, etc.
Outside of metal, I'm very picky and at the same time not... it's strange. I pick wierd stuff from all corners of genres of music to covet, but other very similar artists, albums and songs I could care less for. I love Zeppelin and the Who, but don't really care for Cream or the Stones. I like a lot of old CCR songs, but get annoyed by the Eagles. I like the Fastball's "All the Pain Money Can Buy" record (TOTALLY commercial pop music) ... but find similar pop bands annoying. I enjoyed Pearl Jam and STP but never really liked Nirvana. I like Great Big Sea but would never listen to any other Canadian folk act. I think Nick Drake's Pink Moon album is great, don't care for his other ones at all. I like the first Enigma record (this one had to get drilled into my head mostly against my will, heh.), find the other ones boring. Orff's O Fortuna is really cool as is Wagner's Ride of the Valkyries ("Kill the wabbit, kill the wabbit..."). I like Wierd Al's polka songs. The old Soviet National Anthem was great. The Beatles rule. Simon and Garfunkel's "The Sound of Silence"? Sweeet.
(And then the ULTIMATE in genre defying) I love Frank Zappa's Rock/Pop/Jazz/Satire/Classical/Spoken Word/Sea Shanties/Blues/Big Band/Odd Noises/Just plain undescribable stuff from the 60's and 70's... but can't stand his 80's stuff.
To be honest, I think that hearing new music, that viewing any art or even crap commercial entertainment can't hurt. The more influences you have to draw on, the more creative and original you can be. Stealing from one is plagarism. Stealing from many is research.
Finally (to expand on a thought above), discerning between art and entertainment is important. It's the primary difference between say, Opeth, and, say Titney Spheres. Opeth is Art. Spheres is entertainment. They're both technically music, but exist on two wholly spereate planes. Same thing with anything else. Movies: Schindler's List is art. Pearl Harbour is entertainment.
I wrote a lot. Thank you for your time.