When I was younger I was more impressed with the quick cutaway shots, loud music, funny lines, and action sequences. I grew tired of going to see a movie only to learn they showed all "the good parts" in the trailer!
However the older I got and the more I understood marketing I started to read more reviews from people who posted their thoughts online. I also take into account who is in the movie as well as those who are behind the scenes such as the director and producer. If I've enjoyed their body of work for the most part then I'm far more likely to give a movie a shot.
As for getting the "unexpected" that is what I'm hoping for! A surprising twist that I did not see coming and yet it makes logical sense.
Two examples that stand out in my mind are "The Sixth Sense". I thought I was going to see a horror or spooky movie but it turned out to be something much more better. The twist at the end with Bruce Willis' character caught everyone in the theatre by surprise.
When I went to see "Million Dollar Baby" it was primarily to see Clint Eastwood and Morgan Freeman. I thought it was about a gruff boxing manager and upstart woman boxer. However it turned out to be something much more powerful about father & daughter relationships as well as life and death decisions. Everyone in the theatre was emotionally moved.
I suppose it is difficult to advertise enough to entice people to come see the movie while holding back the best part. To me the most disappointing thing a movie can be is "predictable". If I've figured out every plot twist in the first 10 minutes I know it will be a waste of my time. Another let down is when there is a contrived ending with a bunch of plot holes in a horrible attempt surprise the audience.