Shaman King is a shounen manga (later adapted into an anime) written by Hiroyuki Takei, that chronicles the adventures of Yoh Asakura and his friends as they attempt to win the Shaman Fight, a tournament style brawl directed by Native Americans that is held every 500 years. Shaman King's exposition seems like a fine topic for a shounen manga series, what with all of the giant fights that can be held together by its tournament like setup. However, the execution of the series falls flat, and there are a number of faults and design choices that contribute to the below-par rating that I'm assigning it.
Shaman King's greatest fault is no doubt the absurd length of its fight scenes. An example of this happens in the tournament saga of the story; in what was supposed to be a curb stomp battle by the resident big bad, the big bad spends seven to eight chapters going on about the great spirit and taking forever to just kill some fodder. Keep in mind that this snail-speed pacing is prevalent throughout the entire story: expect every single fight to take anywhere from five to thirteen chapters!
If the pacing of the drivel isn't enough to convince you, there's the assignment of power levels to deal with. In shounen manga, quantized power levels don't work out well, because it's tough to put a realistic cap on power. Like in Dragonball Z, the "furyoku" power levels hit the mid-millions before the end, which directly kills the atmopshere of realism that any story needs to convey its main points to the reader.
When you add in the shallowest characterization possible (that makes bowdlerization look elegant), the fact that one spear, sword, or explosion must be involved in every battle, and that death means nothing in this series (as one can simply come back to life) and you end up with a below-average story. A story so below-average that it makes FFVII: Advent Children look good by comparison.