IIRC the comic book series did a bit more with flashbacks / the history, which the movie kinda skipped over / summarized in a montage.
Which, to me, is what a movie adaptation of something like this should probably do where possible. That "The Times They Are a-Changin'" sequence was brilliant, and the best part of the movie.
It was also the last point the movie bothered to be a movie, but that's getting away from my point (but also not really).
I do think the flashback about Jon Osterman's accident that turned him into Dr. Manhattan could have been fleshed out a tad (or at least done without the voice of
the most boring narrator to ever blight the cinema, holy fucking shit) -- but what I would have probably done there, rather than beginning the movie with the Comedian's fight/death scene, is use those three and a half minutes (maybe a little more, if needed) to instead introduce Osterman's story at a steady, engaging pace. Begin with him on his ninth birthday receiving a clock from his father who tells him that time has weight and power -- but then show him at 16 hearing of the atomic bomb and watching his father throw a clock's innards out a window because the Theory of Relativity has made time meaningless (which is ... you know ... Dr. Manhattan's whole schtick).
Anyhow, show us the rest of what happens to him (going to Princeton, attending Einstein's lecture, becoming a physicist, meeting Wally Weaver and Janey Slater), and end this sequence with Osterman's accident, death -- and sudden resurrection (which would actually matter at this point in the narrative, but was meaningless where we got it in the existing adaptation). Then cue that amazing "The Times They Are a-Changin'" sequence ... and
then implement all the other changes I would make.
I could go on for a long time yet, but I will stop there.