I think that one of the things that helps Patrick O'Brian's Aubrey and Maturin series transcend its genre is the skillful embedding of the main character's lives in their world. In most Napoleonic-era-military genre fiction the characters are seen through the events of their lives as members of the military establishment. For example, all of Rifleman Dodd's actions in Rifleman Dodd and the Gun by C.S. Forester concern his military mission. Aubrey and Maturin, however, often do things that are not part of their military life: music, natural history, local politics, and so forth, and places them in a wider scope than many characters from the genre.