Believability helps. Even when the plot is fantastical, as in, say, "Spiderman," it affects me more if only one thing is fantastical, but everything else in the movie is believable. People in the movie should behave the way people really would behave in the situation presented.
It also helps for the plot to not be too repetitive.
Another thing that helps is for there to be some sort of point. For instance, I like action in a movie, but not if there is only action and little other plot.