When I left the theatre, I felt full. That sounds stupid, but there wasn't anything I felt I missed out on or any major things I would change. It had it all: laughs when they were needed (not as concentrated as those in the Avengers), more action than you could ask for, scenes filled with nostalgia and emotion, and an extremely compelling plot filled with twists. A new character shows off some really cool new gear (I won't spoil it), and old characters are developed (Black Widow for sure). Though there are some cheesy lines (it's a Marvel movie, what do you expect?) and Nick Fury makes some questionable decisions (that was a really nice eyepatch!) this movie was such a fun ride and I left with a grin on my face. It exceeded both it's previous installment and my expectations. Go see it - if you like Marvel movies, odds are this will be one of your favorites. Just be careful not to get too patriotic at once and impulse buy a bald eagle.
P.S. - If you're seeing this in theaters, be sure to stay for 2 post-credits scenes featuring Quicksilver and Scarlet Witch!