A story by itself is never interesting. It is interesting because it is told in a certain way, as held by someone in a certain way in their minds. ZSJL does exactly that. You may not like slomos and long runtime, that's understandable. But a work of cinema is about communicating the conviction of characters and reality of the story and helping the audience experience it. It did that for me.
Better character development, a story well-knit together, a menace of an adversary with a purpose and the coming together of people against a common adversary are all that I loved in the movie. I left convinced that it was a story worth spending 4 hours on. I left convinced that there was another way to imagine superheroes than the formula MCU has been using to churn movies over the decade. Granted its dark, but it's no less entertaining. I remember feeling something similar when I watched the Watchmen - director's cut. Long? Yes. Unmoving? No!