Something else that was supposed to be a surprise? Tony Stark's appearance on the post-credits scene from The Incredible Hulk. This ad opens with that scene:
If you're doing an MCU marathon, you can just watch this and save yourself 112 minutes.
Marvel knew better by the time Captain America: Civil War rolled around. Sure, they still spoiled important moments like War Machine getting shot down and the final Iron Man vs. Cap/Bucky fight, but hey, at least they used misleading editing to fudge the details! (It looks like Bucky shot down War Machine, and Tony's angry over that.)
Finally, the Japanese trailer for Avengers: Age of Ultron, doesn't just give away the twist about Hawkeye having a family -- it's practically the centerpiece of the trailer, as if they'd decided that Jeremy Renner's dull farmhouse life was the movie's main selling point.