I liked it. I was nervous that having so many characters would make it a mess (The X-Men movies have never been able to juggle the team effectively; that’s why I would rather see the X-Men as an ongoing television series rather than Film) but the Russo brothers found a way.
They made Thanos the protagonist. It was a brilliant play and took it straight from Bond. In the Bond films the villain is often the protagonist. Bond is the foil to the villain, not the other way around. The Bond villain’s goal and tragedy is the central theme in those films; Bond exists to move the plot along. It’s always worked.
I will say that I agree with another poster that the emotional impact will ultimately be negated in the next film. The ending was great on its own but I foresee it being cheapened with the blanket revivals and time travel the next films will focus on. I would have rather they killed off some of the older characters (especially Iron Man) for good and went with a more dystopian vision going forward.
My question is this. What comes after all this? Now that Disney is going to own Fox this would be a good time to integrate the Fantastic Four. For the post Thanos universe I would LOVE to see a proper Dr. Doom be the ultimate antagonist. They did that several times in the 616 universe in the comics; Doom is often the manipulator behind the scenes and when revealed is thrust into combat with the entire Marvel hero line up.