One Piece is not complex at all. While it deals with deeper issues than Naruto, ( slavery, world government, racism, classism and the definition of justice ) Oda keeps things simple for the most part.
And that is almost the same with Nurahiyon No Mago and Negima. They tackle with issues, each series have it own issues to deal with and the depth of the character personalities.
Sekirei is more about boobs, relationship between men and women, what is love, lost and death. A lot of people die in that manga.