The main problem with the movie Oppenheimer is that nothing actually happens. It's supposed to be a biopic about the director of Los Alamos and the Manhattan Project, but the movie doesn't spend enough time focusing on that. Imagine a 3 hour movie about Tiger Woods that spends only 1 hour at a golf course, most of which is him walking around from hole to hole, and with only one actual shot the entire movie. Oppenheimer is only known for his work on the atomic bomb during World War II, so he's really not that interesting without that context.

From a storytelling perspective, the movie fails because there is no character development whatsoever. That's fine in action movies like James Bond or Mission Impossible, but it doesn't work in a 3 hour drama.

From a context perspective, it's hardly evident what was going on the world at the time, and what the stakes were. I don't think this is supposed to be a WW2 movie, but it would have helped the movie if it had been more of one.

The score in Christopher Nolan movies is usually top quality. The score in this movie seems like it wasn't written for this movie. Many scenes of people talking in bland conference rooms or hallways are accompanied by intense music written for a bomb defusal. It felt like a cheap way to try to pump some action into a film where nothing happened.

If the movie had been 2 hours, or even 2 hours 20 minutes, I would have tolerated it. As it is, it was a movie that can be watched on and off over the course of a few days without hating it. And I don't hate it. I just don't think it lived up the hype, and I think it's perfectly fair to say it's overrated.