all Opeth albums were in Swedish? try to remember how you first discovered them, and think if it was all in swedish instead. For people who speak Swedish, this question doesn't really matter.
It would be cool. I kinda like it when bands use languages other than English in their music. But either way I would have found them equally as impressive. Doesn't matter to me much.
i think it would be a set back. obviously since english is an international language, and it's much wider than swedish. and it would appear opeth needs a wide language.
I'd probably like them as much, but it would have taken me a long time to start buying theie CDs and I probably would have bought them with much slower pace. and I wouldnt' be able to "sing along in my head" too much. :[