I have two major problems with the actual screenplay. First is the handling of Black Widow. While I think everything they did resonated with the character and was true to the writing and development invested, I don't think the circumstances surrounding the plot choices were correct. The thing that irked me most was the capitulation to the current trend of removing initiative from male leads and transferring it to female leads. I am sure for some people this makes me appear sexist and I'll just have to make my peace with that. Anyone who has bothered to get to know me and the story I've been writing for twenty-seven years, should know sincerely that I value the female initiator and adore stories of feminine strength. And I do for deeply personal reasons.
Second is the muddled mishandling of time travel as a concept. With some non-committal hand-waving at time travel being paradoxically confusing, the writers proceed to present their audience with a hypothesis of rules before completely breaking all of them with no discernibly predictable result. Whether this is because they really want time travel to be a mystery and there is some background cannon to the functions of it or (far more likely) they simply don't want to explain their choices and prefer to use time travel as a McGuffin of the highest order, the temporal narrative is self-contradictory at each turn. This requires the audience to either rest comfortably in accepting confusion or to disengage critical thought. It's annoying, but it certainly doesn't ruin the story.
In summary, this was not a perfect film, yet it was intensely enjoyable. I highly recommend it.