Saul forfeited the crown of Israel for committing two sins. First he did not destroy the Amalekites. Secondly, he presumed to offer blood sacrifice.
The various kings of the Old Testament, it seems, committed far worse crimes (idolatry comes to mind). Why then was Saul divinely deposed?
The sin of Saul was not merely the presumption of priesthood. David would later take on priestly attributes, such as wearing a priestly ephod and modifying the Temple cultus. I'm not entirely sure, but it seems that Saul's chief sin was that of kingly presumption. Perhaps we should look back to Moses' "venial sin" of striking the rock twice and thereby losing access to the Promised Land. Charter leaders of Israel are called to obey perfectly. These kingly leaders are so associated with God that any sort of presumption becomes usurpation of the place of God as "true monarch".
I may be wrong about this, so I'd love for anyone to provide some fresh insight on the matter.