At the end of The Avengers, the heroes go their separate ways. However, it is understood that when the world needs them again, they will answer the call. Their parting is temporary because now the world knows of their existence. Fury is keeping tabs on them so that when the time come he can bring them back together.
There is no such future for the Defenders. Showrunner Marco Ramirez compared the end of The Defenders to a group of people who get in a bus accident. The characters have experienced something traumatic that brought them together, but there is no promise of a future.
The events of The Defenders have strengthened the connections between the characters that already existed as well as building new ones. There is the tease of lasting friendships especially for the Heroes For Hire Luke Cage and Iron Fist, as well as a potential rekindling of a relationship for Luke and Jessica. But the feeling that these characters will continue to fight together does not permeate the end of the series. All the characters (especially Jessica) seem content to go back to their old lives.
Overall, the Phase 1 Marvel films have a relatively similar styles. Tony may get rock music while Captain America has the Big Band of his era, but they all feel as if they exist in the same universe. And The Avengers fits into this universe seamlessly. It isn't something totally new, but an extension of the existing films.
One of the strongest parts of The Defenders is how it feels like a puzzle. Pieces of all four shows can be distinctly felt throughout their singular series. There's the music of Luke Cage, the witty banter of Jessica Jones, the classic hallway fight scenes of Daredevil. The distinct color palette of each character is reflected. When Matt appears he's bathed in red, Jessica gets purple, Luke is all in yellow, and Danny is green. When the characters interact their colors blend like the red light coming in a purple bathed room when Daredevil and Jessica fight together.
The style choices of The Defenders reminds viewers that these characters are all stars of their own show. Their worlds may be coming together, but they do not have to sacrifice their personalities or the strenghts of what made each of their shows great. At times this means the stronger series like Daredevil tend to stand-out more. However, in the end, all the shows are made stronger in coming together (like the characters themselves).
Whether it is big, epic battles or darker, grittier fare, both The Avengers and The Defenders each have something for comic book fans to love. The Avengers are the classic superhero team who fight the big bads and save the day. The Defenders represent street level New York City smaller stakes and more intimate stories. The fact that they aren't alike is what makes them work. The world needs more than one kind of hero and not everyone can wear a cape. Sometimes a hero just needs a leather jacket and some attitude.