I think the Black Widow/Hawkeye thing was believable. They had a strong non-romantic bond. It's totally possible for friends, especially friends who have been through a lot of shit together, to love each other more than their spouses/family/whatever. Plus, Hawkeye's family was dead during that scene. So at that point in time, Black Widow was probably the greatest loss he could suffer.
The cap thing.... yeah.... it bent their own rules more than a little. But it's forgive-able. They could have done it a different way, but it gave them a means to pass the shield on to the new Captain America.
Tony's use of the stones.... yeah.... but upping a character's resilience to stuff like that for their final noble sacrifice is tradition for this type of story. So whatevs.
Overall, the film didn't disappoint. I think it did justice to all the build-up and hype.
My only nitpicks I can think of right now:
- Maybe I missed it, but I feel like there should have been some significant point of sacrifice or growth where Cap became worthy of Mjolnir. It was a really cool moment, but it would have been much cooler if you knew how he earned it... besides just being ultimate good guy Cap, which he always was but didn't enable him to wield it before.
- Thanos..... the Thanos they fought in the first film showed respect for his rivals, and pursued his goal with a strong will but you could see reluctance in him too. There were moments where it seemed like he felt tough-love compassion for the people he was fighting against. He thought he knew what was best for them, but sympathized with their passion in opposing him. His past personality was much more brazenly arrogant and sadistic. Which is fine. He could have grown in those between years. It just felt whiplashy. I feel like that, and explaining his ridiculous strength, deserved more development. I really feel like we deserved to know why the fuck he alone among all of his race is so mighty and nigh indestructible. I'm thinking Asgardian history includes them beating the titans in warfare, right?