When development started on the show in 2008, there was concern about launching the show with Captain America as the leader of the team. A great deal of research showed that Cap met a lot of negativity outside of the United States. So, to give people a chance to know the character, it was decided to have Iron Man, whose popularity skyrocketed at the time of development due to the Iron Man (2008) film, lead the team. Most importantly, show creators wanted to mirror the original Stan Lee and Jack Kirby run and form the Avengers using the original five members, having Cap join later.
In terms of tone and style, the series is based primarily on the original stories by Stan Lee and Jack Kirby. However, it also utilizes material from all eras of the comic's run and other sources such as the Marvel Cinematic Universe.
The producers loosely considered the live action films Iron Man (2008) and L'incredibile Hulk (2008) to be Iron Man and Hulk's background for the series. For the Hulk there was the stipulation that the final scene in that movie is what gets the Hulk/Banner relationship to where it is and a more intelligent incarnation of the Hulk. Also, the Abomination has a British accent as he did in the film.
For season two, many of Avengers characters get new costumes. Iron Man gets new red and gold armor, Thor's new costume is the Olivier Coipel version that inspired the Thor (2011) movie, the Skrull who impersonates Captain America gets a new uniform that is more like the ultimate version with a new energy shield, the Hulk gets a pair of shorts that stretch with each transformation, and Black Panther gets a cape.
In the comics Nick Fury led the Howling Commandos. However, since Nick Fury leads SHIELD in the early 21st century, many years from World War II, they are led by Jack Fury (the father of Nick Fury).