The arrival of the first teasers (domestic and international) for Star Trek Into Darkness was met by many critics with a mix of excitement and worry. (I'm basing this on totally unscientific but basically sound info gathered from Twitter.) Excitement because the ad makes the film looks like a solid sequel to the fantastic 2009 film, and worry because the trailer packs a certain grimness that seems at odds with first film's use of humor to lighten the drama. The movie in the ad looks like a slick epic, but not quite the one people were expecting.
The problem is that a trailer is an ad, and ads are designed to do different things than movies. A movie is meant to take you on a journey and elicit a specific set of emotions; an ad is meant to pique your interest and keep the movie on your radar. An ad's entertainment goals are different than a film's, and it's important to remember that. Trailers aren't going to look like the films because they aren't trying to. They have totally separate purposes.
Take a look at the full trailer for 2009's Star Trek. It's bombastic and loud, with snippets of dialogue taken out of context and laid over establishing shots that fade in and out, or over action scenes spliced together so tightly there's no hope of sorting them out. In other words, it looks exactly like every other trailer for modern genre movies. The trailer is only vaguely related to the film it's meant to advertise.
It can be fun to watch trailers and get excited about coming attractions, especially for a movie you've been anxious to see. But it's vital to remember that trailers need to be watched carefully, and always with the caveat that they will bear little if any resemblance to the final film product. Trailers are just the carnival barkers trying to catch your eye. The real ride is something else.