Any casting decision and armor image for the Iron Man movie, which proved to be very good omens. Particularly, Robert Downey, Jr.as Tony Stark: everyone was nodding when they heard that one announced.
Marvel also had a go at this with what can only be described as a preemptive Fandom Rejoice moment with the film's post-credits sequence - Nick Fury, the Ultimate Marvel version (based on Samuel L. Jackson), actually played bySamuel L. Jackson. Though, part of the agreement to use his likeness in the comics was that he'd have exclusive rights to play the character in films.
The briefcase suit. Seriously, the briefcase-Transformation Sequence has got to be one of the most salivating Technology Porn moments in the history of film. It was so awesome that when it was revealed in one of the trailers, bloggers the world over had a nerdgasm and used that screenshot for their posts. Let's give a hand to the SFX people for pulling off something that's been done for forty years in the comics but seemed impossible to do on film: actually make a suitcase armor realistic.
Gee, I wonder what the stinger will be for this one and if it can match Nick Fury.....wait, THE HAMMER!
The smartest thing the producers have done, and probably ever will do, for the movie was to hire Joss Whedon as co-writer and director.
Like with the Heimdall example above, many people went What The Hell, Casting Agency? when it was announced that Mark Ruffalo would be replacing Edward Norton as Bruce Banner. After the film was released? No regrets.
...Was that a Skrull in one of the trailers?
They turned out to be Chitauri instead, but it was still pretty damn awesome, so nobody's complaining.
Speaking of trailers...this. Particularly the moment at 1:53 when the camera circles the Avengers standing together, ready for battle. Every Marvel fan can probably remember sitting there, seeing this trailer for the first time, staring unblinking for two whole minutes as the words "this is going to be the greatest thing ever" repeat themselves over and over in blogs across the globe.
So The Avengers was pretty great, huh? It's a shame we never found out who the master of that creepy guy who gave Loki his army was, but oh we-...oh, here he is! Wait, did he just mention "courting death"?! Oh my God, is that Thanos?! HELL YES!
Then it was learned that X-Men: Days of Future Past also has Quicksilver, so a deal was struck that Quicksilver can be in both as long as the films won't reference each other.
Iron Man 3 made fans fear that Robert Downey Jr. wouldn't appear in the Avengers sequel, but he signed another deal with Marvel Studios. That same article assures fans that they can also expect to see him in The Avengers 3!
The full title of The Avengers 2 excited fans by revealing a new villain.
The trailer that revealed him is just epic. First there were the voice-overs while a helmet was being made, and at first it looks like Iron Man's helmet. But then wait...it's being dented and melded into something. Camera pulls back to reveal Ultron's head, and well...just listen to the screaming fans.
James Spader will be voicing Ultron. No rumors. It's confirmed by the official website itself.
Now Paul Bettany is the Vision! J.A.R.V.I.S VA seems to be getting an upgrade.
Baron Strucker is appearing, and he looks downright identical to his comic book counterpart.
The picture in question comes from "To Steal an Ant-Man," the episode which details how the original Ant-Man, Hank Pym, found a successor, Scott Lang. This provoked speculation on the movie possibly showing more than one Ant-Man.
Two Ant-Men in the film seems even more likely now that Hank Pym has been confirmed to appear, though nobody's said if he would be Ant-Man or not. And who's playing him? Michael Douglas.
During the end credits of "Marvel Studios: Assembling a Universe", we see what appears to be footage of Ant-Man.
For the people who were outraged by the Mandarin twist, Ben Kingsley's One-Shot reveals at the end that that there is a real Mandarin, who is possibly going to execute Trevor Slattery for impersonating him. Bonus points for the Justin Hammer cameo.