Marvel's Inhumans premieres on September 29, it is an introduction to the Inhuman Royal Family. The characters are based off the comic book characters who appear in a number of Marvel comics like the Fantastic Four and X-men.
Marvel's Golden Age
Marvel has done a good job of bringing comic book characters to the big screen and television. Their winning streak began with the movie Iron man and continued with Thor, Captain America and of course the Avengers. Since then the Marvel studios have been releasing one hit after another, they continued their winning streak with Ant-man and the Guardians of the Galaxy.
Even their TV shows are very popular (although not as popular as the movies). Agents of Shield has a loyal following and Netflix surprised everyone with their version of Daredevil.
The right ingredients
Marvel is able to retain the essence of the comic book characters and adds a lot of humor to their movies unlike DC who tend to keep their movies dark and gritty.
Marvel has created a universe that crosses over from the movies to the TV shows. Events that happen in the movies also happen in the TV shows creating the feeling that they are all connected and part of one big universe. There are also crossovers where characters from the movies appear in the TV shows.
DC on the other hand uses different actors for their movies and TV shows. The best example is the fact that they use a different actor for their character the Flash in the upcoming Justice League movie instead of the actor in the very popular Flash TV show, in my opinion this is a missed opportunity on the part of DC.
The not so good
Marvel sold some of the rights of their popular characters to FOX, in particular mutants like the X-men and the superheroes Fantastic Four (I'll come back to these later). This is why the word "mutant" can not be used in TV shows like Agents of Shield or Movies like the Avengers. This is a problem because mutants are an important part of the Marvel Universe. The Maximoff twins Quicksilver and Scarlet Witch who appeared in the 2nd Avengers Movie are mutants (in the comic books) but in the Marvel movie they just had powers.
To make matters worse, Quicksilver also appears in the Fox Movies of the X-men kind of "breaking" the Marvel Universe continuity. To make things even worse, Fox has not been able to achieve the same level of quality as Marvel with their versions of the Comic book adaptations. The Fantastic Four X-men and Wolverine movies never achieved the popularity of movies like the Avengers or Guardians of the Galaxy (and with good reason). It can even be said that FOX really made a mess of things.
Fatigue setting in
Marvel is looking to expand the Marvel Universe by introducing lesser known characters. They had a lot of success with Ant-man and Guardians of the Galaxy in particular. But lately it seems that the quality of the Marvel productions seems to be slipping. Iron Fist was not as good as Daredevil, Jessica Jones or Luke Cage. Dr. Strange not as witty as Guardians of the Galaxy or Ant-man.
Marvel is reaching a point where all they have to show to the public are superheroes who will not be recognized by everyone and this might hurt the popularity of the franchise
Marvel's latest
Inhumans were introduced in the (movie) Marvel Universe in Agents of Shield. Chances are most people never even heard of the Inhumans.
It will be difficult for Marvel to sell a Royal Family of super powered beings living on the moon. My point is that the Inhumans are rather an obscure kind of superheroes (You can even question if they are superheroes at all?).
The Inhumans in the comic books own their fame because of their affiliations with the Fantastic Four and the X-men. But guess what? The rights to those characters belong to FOX and there is no chance that any of these characters will appear in the TV show.
So the Inhumans will have to gain popularity on their own but my guess is that will not happen.
Agents of Shield had a rough first season even with the addition of Coulson from the Iron Man and Avengers movies. Things for Agents turned around after the Captain America: Winter Soldier movie. After the events in Winter Soldier carried over to the TV show things just kept getting better and better. Now we even have Ghost Rider doing his thing in the TV show.
But Inhumans has none of these things going for it, that is why I am very pessimistic about chances of this show being popular with a mainstream audience.