My opinion is based on the assumption it is only 2 hours, because we don't know for sure yet. And I fully acknowledge I don't have all the info and if it is 2 hours, it could still be done in a way that I really enjoy. Having said that:
2 hours does sound disappointing. Can they make it work within 2 hours? Sure, it's possible. Do I think it's likely, not so sure. I was hoping for a more epic JL film. I wasn't expecting 3 hours, but 2 hours 20-30 minute range is what I was hoping for.
I've seen enough movies to get a sense of what can be done in an 1.5 hour movie, a 2 hour movie, 2.5 hours, and so on. I also know how many big comic book movies are around 2.20. 20 minutes makes a big difference, spread throughout the film, allowing it moments to breathe, smoother scene transitions, staying longer with certain scenes to allow us to live in those scenes, and thus the movie.
When I saw the possible news that Thor would be 1:50, I thought that would be too short for a Thor and Hulk movie that would have elements of Ragnarok and Planet Hulk, but it seems that the movie was 2:10, and that's a movie with only two main characters who have had plenty of past screen time. It also seems like a movie with a fast pace and light on the plot. And yet it's still over 2 hours.
The part that worries me if it is two hours (and of course this could change when I see the movie) is the possibility that WB would be doing this for solely financial reasons (as in, the suits are calling the shots, damaging long-term success like with BvS and SS) rather that combining business with art, which I think allows for the most success.
I also think there's a false equivalency when comparing it to a movie like Ocean's Eleven when we can be quickly introduced to each character and then watch them as a team in what is a fast paced movie, and meant to be that way. Also, comparing it to horror movies like IT doesn't work either, imo, because horror movies and comedies tend to work better in the 1:30-1:45 range.
With Justice League, you are bringing in characters who each themselves are characters (maybe aside from Cyborg) who can get a solo movie and have fans who find that character their favorite one. I don't want to finish this movie and feel like, "Hmm, that was solid, but it felt like it could have been so much more, and it doesn't feel like it completed this 3 movie arc."
Again, the part that concerns me about it is whether this is WB interference, and if it is, then I think that's a problem and don't trust that. Again, this is all on the idea that the 2 hour running time is true, because this is still speculation at this point.