Joseph Hill “Joss” Whedon (born June 23, 1964) is an American screenwriter, executive producer, director, occasional composer and actor, and founder of Mutant Enemy Productions. He is best known as the creator and showrunner of the television series Buffy the Vampire Slayer (1997–2003), Angel (1999–2004), Firefly (2002) and Dollhouse (2009–2010). He is also notable for his work in film, comic books, and online media. Many of Whedon’s projects, as well as Whedon himself, enjoy a cult status. –wikipedia
Why does he get so much praise? The only thing I know him for is "The Avengers", and yes he did a great job with it... but I feel like too many people know his name and praise him for one film. But perhaps I'm ignorant when it comes to his other work.
He is praised because Buffy the Vampire Slayer, Firefly and Angel are all considered fantastic television shows. He was the showrunner on all three.