I do everything I can to avoid watching trailers. I won't look them up on purpose and when a youtube video starts with a trailer I try to skip it as soon as possible. In cinemas I look at my phone and listen to music while the trailers are playing.
It wasn't always like that. I used to look for as much information about movies as possible, watching trailers, stills, interviews,... I eagerly awaited the release, knowing this movie would be one of the best movies ever. More often than not, I was disappointed.
The purpose of a trailer is to get people to come and watch the movie. To make the movie look appealing, they show what the story is about. For comedies they show the best jokes and for action movies they show the coolest action pieces. But most of the times, they give away most of the film with the trailer.
Most films have the same three act structure. When you know the setup of the story you can roughly guess what will happen. Watching the movie becomes a "connect the dots" between the scenes you've already seen in the trailer.
For example, the trailer for 'The Dark Knight Rises' (Spoilers ahead!)
The trailer shows a lot of the story happening. You know the football field is going to be blown up and you know Batman and Bane are going to find surrounded by other people in the snow. That's not that bad, it leaves out some major plot twists. But there's also the scene when Bruce Wayne is in the prison and asks what they are chanting. This scene happens in the movie after he tries to make the jump a second time. I knew he wouldn't make the jump because we hadn't seen that scene yet.
Another one: 'Chronicle' (spoilers again)
I like the start of the trailer, but they even show the big battle at the end of the movie. The trailer even ends with a full rewind of the entire movie.
I'm not saying trailers shouldn't do this. If you want people to watch your movie, you need to show all these cool bits to make your movie look appealing. But I don't let my choice of movies be determined by a trailer. I choose the movies I watch based on other things: the director, the cast, the writer, the source material,...
Sometimes it's better to stick with the trailer and not watch the entire movie. A mediocre comedy will probably show all its best jokes and an action movie shows all the coolest sequences. The trailer condenses all the coolest bits in 2 minutes so you don't have to waste 2 hours watching the movie.
An example: 'The Expendables 2'.