I can understand why the two Infinity Stones Thanos got off-screen weren't going to pose a particular problem. But of the ones we see on-screen, there seem to be a couple that were rather too easy - first, the Tesseract:
Why did Loki give it up to save Thor? It's not like they're best buddies, and stopping Thanos from killing half the universe is surely worth a little collateral damage. Given where Loki had it stashed, if Loki had held firm then Thanos would never have got it. Also, Loki isn't a hero with principles, which could be an argument for why Gamora gave up the Soul Stone.
Why was it all so relatively effortless?