A few thoughts about the preview and the production of the movie in general:
1) I'm disappointed the great composer Alan Silvestri is not returning to score the sequel. I liked the theme of the first film. Replacement Brian Tyler is a competent musician himself, and I hope he can create a memorable theme.
2) As with all other sequels in a series, this movie looks significantly darker than the original. I have faith that writer/director Joss Whedon will keep his wit and whimsy this time around as well, especially when the shadows grow darker.
3) I found it interesting that Captain America himself, Chris Evans, is given fourth name in the credits when he was listed second in the first film. Does this mean he has less screen time this go-around?
4) James Spader's voice for Ultron more closely resembles his own voice in this trailer, whereas in the first trailer there was a more mechanical tone to it. Perhaps they are disguising it more because they want his voice to be more surprising and frightening when the film actually releases (much like Bane from "The Dark Knight Rises")?
5) Excellent to see Don Cheadle (aka War Machine, Colonel James "Rhody" Rhodes) amongst the "Avengers" cast members this time around.
6) "C" is a very popular letter amongst names of the actors in the Marvel movie universe: Chris Pratt (Starlord), Chris Evans (Captain America), Chris Hemsworth (Thor), Christopher Eccleston (Malekith), Don Cheadle (Major Rhodes, aka War Machine), Clark Gregg (Phil Coulson), Cobie Smulders (Agent Maria Hill), Bradley Cooper (Rocket Raccoon), Dominic Cooper (Howard Stark), Chadwick Boseman (Black Panther), Benedict Cumberbatch (Doctor Strange), and a bunch of smaller actors in smaller roles.