This page contains spoilers of various movies. However, it doesn't contain anything that the trailers of those movies wouldn't spoil already, so I suppose it doesn't matter.
I like movies which surprise me. It doesn't have to be a huge surprise that leaves me in awe, but even small unexpected surprised are ok. Of course big surprises are the best. When I go to watch a movie (or rent a DVD) I basically don't want to know anything about the movie beforehand. I want everything to come as a surprise.
I really, really hate trailers of movies. Most of them just show way too much. I don't want to know so much about the movie beforehand. In the extreme case trailers reveal relevant plot surprises.
When I went to watch Jurassic Park 2 I somehow had succeeded in avoiding seeing any trailer of it at all. I had absolutely no idea how it would be (of course I knew the premises because I had seen the first movie, but besides that I just knew nothing about the second movie). Many people might not consider the fact that the T-Rex got loose in a big city a big surprise, but for me it was an awesome experience. I was absolutely not expecting anything like that (well, I was young and naive back then and things like this came as a total surprise to me) and I was completely thrilled when it happened. It was an awesome experience!
If I had watched a Jurassic Park 2 trailer it would have completely spoiled this surprise and my watching experience would have been considerably diminished.
Ok, that was not even a bad spoiler. In some cases the spoilers given in a movie trailer are beyond incomprehensive.
For example, I also somehow succeeded in not seeing any trailer of the movie "The General's daughter". This movie is a murder investigation thriller full of surprises and unexpected plot developements. Surprising things are little by little revealed about the murder victim.
When I later saw the trailer of this movie I was absolutely horrified. The trailer gave away every single major plot surprise in the movie! Except for the identity of the killer, nothing else was left out of the trailer! I was completely horrified at how badly the trailer spoiled the entire movie. Watching the trailer would have destroyed the watching experience completely.
Another example of a trailer spoiling a major surprise in a movie is the trailer for the movie "First Daughter": The fact that the protagonist's boyfriend was in fact a member of the secret service is directly shown in the trailer even though in the movie this was a major unexpected surprise given very late in the movie. The trailer would have completely destroyed this experience if I had seen it beforehand.
Trailers not only give away major surprises in a movie plot, but they often also give away all the major jokes in comedies. For example the trailer for "There's something about Mary" is a good example of this. I actually watched this trailer because I thought that I would not see the movie anyways. I got to regret this decision because I later did watch the movie. I counted exactly one major joke in the movie which was not already shown in the trailer. The movie was basically completely destroyed by the trailer. The trailer was basically the movie reduced to 1 minute, with all major jokes included. There was basically nothing funny about the movie anymore after that.
One could argue "if you don't like trailers, don't watch them". However, if I go to a movie theatre I have no choice! Ok, I can close my eyes (which I actually always do), but I can't help hearing the trailer anyways. Sometimes just the soundtrack of the trailer gives major spoilers.
One really bad example happened to me recently. They showed a trailer of the movie "When a Stranger Calls" and even without watching I could just deduce the majority of the movie from what I heard, and I just couldn't help it. And what's worse, the soundtrack really clearly gave away something which is most probably a major surprise in the movie! And I couldn't help but hear it!
Ok, here comes the spoiler: Deducing from what I heard, there was some babysitter in some secluded place which starts receiving harassment phone calls, more and more of them. And then, at some point, the police calls her and tells her that the calls are coming from inside the house! At that point I was completely outraged. "Gee, thanks a lot! You just gave away a probably very major surprise element in the movie and I couldn't help but hearing it."
I really hate going to the movie theater because of the trailers. They just suck. They ruin my movie experiences.
Sometimes also when I watch TV, trailers come as a surprise. There are certain movies where I simply don't want to know anything at all, no images, no sounds, no nothing at all. This is usually the case with big movies (such as the Matrix sequels, the LotR sequels and so on): I don't want to hear or see anything at all about them. I want to see them in a completely pristine state, I want to be a complete and pure virgin when I see them, I don't want to be spoiled in any way about them. Yet sometimes they just suddenly and without any warning show trailers on TV. Even though I quickly stop looking and change the channel or anything, I still get to see at least some seconds of the trailer, especially if it has been camouflaged as something else than a trailer for the movie at first.
Often even movie posters give away some details about a movie which I don't want to know anything about beforehand, which is also a bit irritating. (For example the movie posters of "Batman Begins" gave away his suit, something which didn't want to know beforehand. A small thing, but it still irritated me.)
I really, really hate trailers. They should put them where the Sun doesn't shine. Deep there. With a pick axe.
I have to admit, though, that not all trailers are bad. Ok, scratch that. I don't remember seeing any good trailer whatsoever. However, there are a few so-called movie teasers which are good. Teasers are usually much shorter than full trailers, and often being separately made, ie. containing scenes not in the actual movie. Two examples:
A Terminator 2 teaser which shows a scene of Terminators being built. (I don't know if this was just a deleted scene from the movie or if it was made specifically for the teaser, but it's cool.)
A Godzilla teaser which shows a pun on Jurassic Park. The slight humorous pun aside, the teaser is one of the coolest pieces of video I have ever seen. It still gives me shivers watching it. If the movie itself would have been even half as good as this teaser, it would have been one of the best movies of the 90's.