Some initial thoughts:
1. James Spader as Ultron just looks so good.
2. The fight sequence between Hulk and Iron Man is going to be epic.
3. It looks like Scarlet Witch will have the ability to control minds, as can be seen from the quick shot of her apparently controlling Black Widow, perhaps she does control the Hulk as well.
4. I think you can see some of the splintering between Captain America and Iron Man start to happen.
5. The Vision, I'm glad they only teased him.
Marvel is very good at hiding the ball on what the consequences of their films are going to be, as in Winter Soldier, so I'm excited to see whats going to happen, glad they didn't give much away in the trailers.