Another interesting case study is Tuvok. He joins up as serves as an Ensign on the Excelsior in 2296, apparently doing very little to disguise his feelings of distaste for the Starfleet life. He then resigns, spends half a decade doing whatever (raising a family on Vulcan mostly), and then returns to the service in 2344, apparently with a greater understanding of what Starfleet can give him in his life.
I have often wondered if, for example, Tuvok would have returned as an Ensign? It seems like officers who resign and then come back seem to pick up pretty much where they left off career-wise.