Question: In the original Avengers movie, after Captain America breaks up the Thor/Iron Man fight, Thor goes to hit Captain America with his hammer, Captain blocks it with his shield and Thor goes flying. In Age of Ultron, they do the same thing intentionally in the opening battle scene, and it flips the tanks and vehicles but Thor is unaffected. Why the difference?
Question: When Wanda uses her powers on Tony, he sees his friends lying dead on the ground. Later, Tony says that what he saw was something that was actually going to happen? Why would Tony believe that the deaths of the other Avengers is something that would actually come true? Wanda was very good at messing with his mind so, she could have made him believe that their deaths would occur.
Answer: What Wanda did was to amplify Tony's existing anxiety about failing to protect his friends and the world. He was probably already having dreams and thoughts to that effect before encountering Wanda - when she used her powers on him, the resulting vision was so vivid, but in keeping with his previous fears, that Tony was convinced it was an actual premonition of the future.
Question: Why did Thor let Cap have some of his drink that he said mortal men can't have. I know that Cap's super soldier serum makes him stronger, but he's still human.
Answer: By mortal men, he probably just means ordinary. Since Cap has super powers, Thor doesn't consider him ordinary (plus the we learn in Captain America: The First Avenger that his advanced metabolism prevents him from getting drunk). Since Thor also relents and allows the WWII veterans to partake in the drink as well, it's safe to assume he doesn't consider it to be deadly for humans to have.
Answer: Cap now has a superhuman metabolism, as shown in the first Cap movie when he was unable to get drunk after Bucky died.
Question: Why was Captain America unable to hold Thor's hammer? He fought to protect his country. He saved the lives of innocent people and wasn't afraid to back away from any kind of danger so he should have been able to lift it. Ultron said that the reason none of them could lift it is because they were all killers but that's really just speculation.
Answer: He can pick up the hammer, he didn't want to upset Thor.
Answer: He's able to move it just barely, enough to impress Thor. It's an enchantment on the hammer that requires a combination of strength and righteousness. He might not be there yet, but he's on the way. Thus the hammer moves more than anyone else could.
Question: What was Steve referring to when he said he'd seen Natasha flirt up close?
Answer: He was implying she had flirted with him at some point.
Not just an implication. We see this explicitly in The Winter Soldier.
What was it?
Natasha is constantly flirting with Steve throughout the movie.
Answer: The first time it was unexpected. After that they realised it was a good military move. The way they did it, they most likely practised and have done it several times before. They said they've been hunting and destroying HYDRA secret bases for three years since the events of the first movie.