In their bio of Castellano, "Boss of Bosses," Kurins and O'Brien list several beefs that Gambino soldiers and associates had against Big Paul:
--He never mixed with or socialized with people under him, preferring to summon a select few come to his Staten Island mansion.
--He dissed Neil by failing to come to his wake or funeral.
--He made concessions to other families that disadvantaged his people.
--He carried on, openly, with his Colombian housemaid, Gloria Olarte, right under his roof with his wife present.
--He involved himself in nickle-and-dime deals that should have been left to lower level people.
--Most important: he cheaped out on his people, refusing to share the wealth.

Kurins and O'Brien also say that the other Dons were pissed because they heard Castellano, on tapes, dissing them. They also feared that he'd rat them out rather than spend the rest of his life in prison where he would be away from his beloved Gloria.

BTW: In "Underboss," Peter Maas's bio of Gravano, Da Bull denigrates Castellano as a mere "racketeer," whereas he and Gotti were real "gangsters."