it's mood music, its emotional....and it has to affect you in ways beyond just "oh that riff is cool" for you to "get" it. i'm sure Agalloch isn't for everyone, but it's one of those things that I find it worth putting effort into really feeling the music they create, what images it brings up, the emotions, it just hits on so many different levels. I also find it to be com personal, like I can't get into it with other people around to any comparable extent to when i'm alone. It just requires a lot more then a casual listen. Not that they are very similar but I often compared it to Emperor's Prometheus, in that while it struck me as good the first time I heard it I really didn't comprehend the extent to how amazing the music was until I had given it several listens.
And for those that havn't heard it, if you can get past The Mantle then definitly listen to Pale Folklore. I find the emotion to be even more raw (such as the 3rd track). Of Stone Wind And Pillor is great too, A Poem By Yeats, besides being an amazing poem in itself, is a really reflective track that fits perfectly. And Kneel to The Cross is just wonderfully blasphemous in an understated fashion that should make any corpse paint wearing black metal band bow down.