First off, a lot of these reviewers keep spoiling the movie...Why can't they talk about their opinions and not the damn movie?....Second, for me, the only legitimate criticism is when they talk about the movie havin' to pay its dues to past and future franchises. Because that literally breaks a movie apart. You're not seeing a movie. Just a bunch of sophisticated commercial of other movies. They should focus on this movie alone and not the others.
Other than that, most people are talking about what they wanted to see and not what they saw. They wanted to see more of QS and Scarlet Witch, more of cap vs stark, not wanting to see monologues, etc.
Edit: It must be hard for marvel trying to please everyone: those who want nods to past films, teasing some future characters, making a consistent universe, etc.