I agree that the movie needed more build-up with Thanos' background.
There were also certain parts of the movie where important plot points happened because, in my opinion, characters were being stupid. Specifically in how Thanos gets certain stones.
I appreciated everything that happened involving Dr. Strange and his Time Stone, including the ending.
In the end, I had mixed feelings. I was glad to have a villain who believed himself to be the hero, and there were certain pieces that I felt were well done, but others that felt less natural or even forced for the sake of plot.