Jason isn't exactly your typical hero. Unlike, oh, say, Heracles, who loves bashing every monster he comes across, Jason has no problem letting other folks do the bashing for him. Pretty much every challenge Jason faces while on the quest for the Golden Fleece is solved by somebody else. Polydeuces out-boxes the heavy weight champ, King Amycus. Zetes and Calais, the flying Boreads, drive the Harpies away from starving King Phineus. Heracles punks down the Earthbound Men, and, of course, there's Medea, whose sorcery is indispensable in Jason's acquisition of the Golden Fleece.
It seems like Jason's real strength lies in his leadership skills. Nobody ever says he's a bad warrior or anything, but his quest is truly achieved through his skills at delegation. Instead of sailing off to get the Golden Fleece all by himself, he assembles the most intimidating team of heroes the world has ever seen. The Argonauts have loads of different skills that will all prove useful on the quest. Jason knows with this super team of awesomeness surrounding him there's no way he can fail. His choices of who to include in his crew of Argonauts show great foresight and wisdom. He's successful in his quest because he knows how to tap into the synergy of those around him and put them the better good of the whole team.