I've been getting a few naysayers who hate trailers because of how much they reveal. While a trailer that reveals too much is probably poorly cut, I, like Pooja, maintain that the story told by the trailer is pretty much independent of the movie, so it almost doesn't matter what the trailer shows. I love that they are miniature movies in their own right, that they have their own structure (intro/beginning, exposition, rising action, climax, resolution/suggestion), and yes, they keep you wanting more.
Case in point: a few years ago there was a contest among students of film editing, and the task was to take a popular film and recut a trailer for it that would make the movie look like it belonged in a totally different genre. The best of the bunch was this, The Shining, recut.
Now it's got a bunch of copycats on YouTube, but this was the original. Total genius.