Ultimately, I think centralization is not the essence of what those of us who want to avoid it actually loathe. We loathe coercion. After all, for whatever reason, we have violated directive #3 (I think - right? That was Stephen's example, right?) because, through some centralization, the community was able to reason with miners and thereby convince enough of them to switch to a broken client in order to save the masses of bitcoiners still on that version, unable to validate the good block that forked them over. There is a kind of coercion in that, but it depended on the cooperation of the miners and isn't something I would call coercion because no rights of anyone were violated or threatened to be violated.
That is semantics. Centralization explicitly refers to the centralization of power in the hands of governments through legal coercion.
That being said, semantics are important...
That being said, the whole concept of p2pool is to further avoid "centralization" of power done not by the government and not via coercion.