There can be no bigger culprit of this than the Terminator trailer which gave away a huge plot twist right in the middle of the second ad. It stated (in multiple ways) and even shown that in this movie, John Connor becomes a half human/half machine at some point and is actually the villain of the new film. If you're unfamiliar with the series, it'd be like learning in the middle of Iron Man IV that Tony Stark has actually been in charge of HYDRA the whole time. Regardless, this new twist does make the new movie stand out more...but seriously? What else is there to see in Terminator V now except for a lot of running, jumping, and explosions? Is this a red herring and there's actually a second twist we haven't heard about yet? Maybe...but I feel like the studios just ruined their own movie to sell tickets. The same seems true with the new Star Wars movie. Even though the teasers haven't shown much, we already have names for all the lead characters, we've seen the returning actors, the villain (cloaked in a hood and only see from behind in the first shot) has now been shown full on...there's not much left to wonder about.
Harrison Ford had a quote about a year ago saying he was glad the original Star Wars weren't released now. "It never would have had the same effect" he said. "You couldn't do it now; everything gets leaked and ruined and spoiled." He's right in a lot of ways. The only way movies become these cultural hits are when they have the freedom to surprise people. Avengers did that by showing that superhero movies could be action-packed but also fun, Avatar did that with it's 3D effects, The Dark Knight reserved it's big twist for the film and didn't give it away, and of course, the original Star Wars came out of nowhere to shock the world (or so I've read). Movies today...all we're really waiting to see are the boring scenes to connect the trailer shots, and hopefully something interesting at the end.