That would be the use of a cute girl who kicks ass. Buffy, River, the cast of Dollhouse, Kitty Pryde in his X-Men work. But I like Whedon quite a bit, so I don't think it's fair to call that one-trick pony material. Angel didn't have a cute girl kicking ass until the end of the last season, and Fred wasn't exactly cute by that point. River only kicked ass in one Firefly episode and then the movie (you can argue that Zoe kicked ass, but she was a different sort of character, she wasn't the cute girl character, she was a hardened war veteran). Dr. Horrible didn't have it. He also wrote/rewrote much of the script for the original Toy Story, and that doesn't fit his so-called mold either. Avengers probably won't fit it. And there are other movie and tv scripts he's written too.

His dialogue is similar across many of his projects, but that doesn't count as one-trick pony material either, like you said. That's just how he does dialogue, he likes banter and he likes to make jokes, and a lot of people enjoy it.

Whedon writes stories that he enjoys, so there are some similarities to some of them, but he doesn't just repeat the same story over and over.