Here is my personal opinion,
As a standalone film, this might be one of the best action/fantasy blockbusters ever made that has little-to-no character development. As a new chapter in the Marvel Universe, it's absolutely solid and perhaps one of my favorites thus far. This movie has one purpose: begin the process of tying loose ends, which it succeeds at wholly. Thanos is a great villain--he has a definite shade of absolute evil, but he is never depicted to truly be insane or out of touch with his humanity entirely. He's not a stupid, 1-D or 2-D villain: he is a 3-dimensional character who for some reason has convinced himself that the universe needs to "be balanced." It's not nearly as developed as I would have liked to have seen: he explains his motivation comes from caring about resources, but there's no personally emotional experience that effected him directly for me to believe yet that he absolutely needed to do what he did. The movie is about 3 hours, yet despite this the content, story, and pacing kept me in the whole time and I wanted more by the time it ended. Having said that, the ending is brutal and heavy so I was, actually, relieved to have some quiet to contemplate. But, it's also Disney so they're going to fix everything in the next film guaranteed, no hesitation in that thought pattern.