American Beauty was Sam Mendes's directorial debut (on film), which is just gut-punchingly impressive. He was a seasoned stage director before breaking into films, but it's pretty amazing to win an Oscar for your first film.
Mendes's films are pretty eclectic, though he describes them all as being alike in that "They're all about one or more people who are lost and trying to find a way through" (source).
American Beauty and Revolutionary Road are kind of alike in that they both portray a stifling suburban culture, but overall—looking at a body of work that includes the war movie Jarhead, the quiet romantic comedy Away We Go, and those Bond movies—it's hard to pin him with a particular label or style.
And that's the way he likes it. Whereas some directors have such a distinct flair that their names become adjectives (e.g., "Wellesian" for Orson Welles), Mendes says he has zero desire for his work to be called Mendesian.