Afro Samurai is a very solid show. It's got all the combinations for success and yet sometimes it still manages to be a bit boring. Which is why it didn't score higher for me.
The Story gets an 8 for being somewhat original, it definitley takes the idea of the Ronin seeking revenge and gives it's own stylistic flares.
Animation gets a 9 because Afro really does look pretty. It always sticks to a pallete of very dark, vivid and saturated colors to give it that rough edge the show definitely goes for.
The Sound gets a 9 because this show is not only voiced by a great actor (i.e. Samuel L. Jackson) but it also contains beats by The RZA from the Wu-Tang Clan. Put those 2 together and it's pretty much as good as it gets.
The Characters only get a 7 not because they didn't have much depth, it was because despite the attempts to make the characters round a lot of them were just unlikeable or very very uninteresting. Afro himself doesn't really say much, but that is made up by his ghostly alter ego who is always filling the conversational void.
Overall I really like watching this show and would recommend highly for anybody to watch.