The teaser trailer showcased the character of Daredevil looking more like Iron Fist from the comics than the actual live-action Iron fist standing beside him. The half-open shirt strategy has not worked, it is time for producers to provide Iron Fist with some sort of uniform--no one is expecting it to be exactly like the comics but they need to at least try. Besides having a crime-fighting billionaire with a one of kind super-powered fist running around mask-less is a pretty bad idea.
2) Team Fights:
The fight scenes should be epic, the thought of seeing Iron Fist and Daredevil back to back displaying their martial art maneuvers is enough to sell the show too many, but they still leaves two other heroes. The Defenders as a whole are holding A LOT of power in their ranks. You have two of the best hand to hand combat fighters in the Marvel Universe teamed up with an invincible Luke Cage and a formidable Jessica Jones, that's not to be underestimated. Hope the show has the budget to keep up with these characters.
1) Satisfying Villain:
These are all different types of characters with very different personalities so for them to team up willingly it has to be a big enough threat to bring them together. Especially when you are dealing with characters like Jessica Jones and Luke Cage who haven’t fully bought into the hero life just yet. The villain or villains will have to formidable to keep the season interesting.