This film is Thanos's. It is he upon whom we concentrate and who gets development, everyone else are bit part players; but, boy some of those bits are fun, and dramatic.
For anyone with even a passing interest in the Marvel universe this is not difficult to follow or complex, as long as you have an understanding of the powers of the stones; the Dr Strange primer being especially relevant.
Any irritations? For me there is the standard trope of simply having hoards charging at each other, at a stroke three thousand years of strategy and tactics is forgotten as we return to the school yard. I can accept that they are not aiming to recreate Waterloo but some nod in that direction would be appreciated, at least by me.
My only hope is that in Part II they do NOT simply wind back the dramatic moments of this film, ape Game of Thrones and have some true emotional weight.