A-B-C
A is for... Angel
I bet many of you thought I'd devote A to his most recent and most lucrative project The Avengers, or the sequel he has in development, but there is plenty of time for Avengers love further down this list. A had to be devoted to Angel, who is the single character who appeared in more hours of Whedon creations than any other character... three seasons of Buffy (including a few episodes here and there later on) and 5 seasons of Angel. Though Angel isn't my most favorite character in the Whedonverse, and it certainly wasn't my favorite show out of Whedon's four (probably ranked in 3rd... maybe 2nd) the influence of this character on both the film and TV world has been vast. Sure, he may not have been the first good vampire on television (see films like Interview with a Vampire) but he certainly popularized vampire romance stories and vampires with souls for a generation of teenagers and adults a like. Whether you think this is a positive legacy or not is up to you, but I would argue, if it wasn't for Angel, shows like True Blood and Vampire Diaries, and the film series Twilight wouldn't have been as popular as they are, and producers probably wouldn't even have the confidence to air the shows or make the movies. Angel proved that women love a brooding dark vampire, and that the horror/supernatural/comedy drama genre works on TV. Plus, the show was still awesome and the character was one of Whedon's most complex.
B is for... Buffy The Vampire Slayer
The greatest television show of all time and Whedon's still most famous (probably) and best project so far, B had to stand for Buffy. Buffy is great genre TV, with great characterization and even more epic storytelling. If it wasn't for shows such as Buffy proving they could be as well loved, critically adored, and truly brilliant on TV you wouldn't likely have the current version of Doctor Who for example. Yes, Doctor Who was around long before Buffy, but Russell T Davies, the person who brought Doctor Who back to the screens has time and time again stated the influence of Buffy on the revamped show. Also, more than Angel, Buffy really kicked off this whole vampire craze thing, and when Buffy was on, vampires were still cool. Buffy was a unique and highly original and creative series with one of the best, most simple premises ever on TV. The show subverted what we expected to see from your average teenage show set in a high school and was so much more than anyone could have ever predicted. I would argue that Buffy really was one of the first truly cinematic TV shows, and one of the first to prove that TV can be just as good, or arguably greater than films itself.
C is for... Comic Books
Not only is Whedon a fantastic TV and Film writer, he is also a very well respected writer within the comic book world... a fact that arguably secured his place working on The Avengers. Whedon himself is a long time fan of comic books, and in 2004 he started writing what is widely considered to be the one of the greatest X-Men comic books ever written, Astonishing X-Men. Added to that, he also wrote a few issues of Marvel's Runaways comic, so he is very familiar with the Marvel world. He is also currently involved with the continuation of Buffy and Angel in comic book forming, occasionally writing a few issues and arcs, but generally overseeing and creating the stories for the books. He also wrote a great short comic book series set in the Buffyverse, but in the distant future, entitled Fray... following the adventures of a future slayer in a very Fifth Element looking time period (she even appears in the Buffy Season 8 comics in an arc written by Whedon himself). If you haven't read a comic penned by Whedon you are truly missing out my friend. Any single example is a great read.