1) The tail of The Leviathan. Remember that call sheet which mentioned where "the Leviathan" came to rest in the street? Well this is probably our first glimpse of it:
Of course, the question which still remains is whether it's an actual creature, or just a ship made to look like one (a bit like dragons on viking boats)?
2) The enemy invaders. There's no certainty that they'll all look like this (the army could include multiple races after all), but the one that we can see reasonably clearly
seems to have Skrull-like ears (either that or it's part of the armour/helmet design), blue skin, three toes, and a green patch (possibly glowing?) on the chest. None of the races in the comics would match these features exactly, but perhaps some artistic license has been taken? In that case, it's concievable that they might be Kree or Skrull warriors after all.
3) The portal in the sky. I think a lot of posters mistook this circle up above, which Iron Man is flying towards, for a mothership. In fact, it's a portal:
So, do the aliens have the technology to open up portals? Or is this the work of Loki? Either way, knowing where the portal goes would likely reveal the identity of those mysterious aliens.
4) What's clipped to Thor's belt? We see the lightning god landing on what looks like the quinjet, but what's that clipped to his side?
Is it his helmet? If so, why's he not wearing it? Has he been on a mission to retrieve something?
5) Triangle arc reactor. This is hardly a big revelation, but we can at least put to rest those claims of inconsistency between Whedon's film and Iron Man 2.
Tony still has the triangle arc reactor (which he developed thanks to his dad's insight) - the only difference is that the casing of his armour seems to change back to the circular design at some point during the film.
If you yourself spotted anything interesting in the trailer, or indeed have any theories regarding these pics, go ahead and post it in the comments section below!