Mushishi isn't a story that tries to blow your mind with unthinkable ideas - it's just the opposite, really. It takes simple concepts that you don't normally think about, and then portrays those ideas in clever ways that then entice you to go climb a mountain and ponder it for a while. Or take a peaceful nap. The music, imagery, and dialog is all very soothing. =]
Every episode is about a really laid-back guy named Ginko, who aimlessly travels around helping people with problems involving colorful creatures that are invisible to most peoples' eyes (sometimes they're glowy insect things, or like gooey parasites, or they blend in with nature in some way, for examples). There is no over-arching plot. There is no mystery, suspense, action, romance, high-level drama, or even a lot of character development (Ginko's in every episode, but the episodes are rarely actually about him).
While I say there's little character development, there are practically no "stock characters" in the entire show. All the one-shot side characters Ginko helps are truly treated as if they were real people. (That wear kimonos.) Oddly enough, at the same time everyone in Mushishi is rather... well, a normal, everyday person. This is not typical for anime, or fiction in general, to be honest - but it works. The situations everyone ends up in for every episode makes the show plenty interesting.
Having said this, I thoroughly enjoyed this series. Some episodes are better than others (as in all animes) but overall there really isn't much I can say bad about the show. Just make sure you're in the mood for something quiet and slow-paced. For allegory, Mushishi's not a series of explosions - it's a bonsai tree.
One freaking awesome bonsai tree.
