IGN:
A Darker Tone
The most immediately apparent change now that director Shane Black has taken over for Jon Favreau is a darker, more somber tone. It seems that Tony's new enemies are systematically dismantling his life in the same way Bane and his followers did to Batman in The Dark Knight Rises. At least Batman got to keep his house and gadgets.
However, it's worth noting that the trailer is focused on only the darker elements of the movie. Footage shown at SDCC 2012 showcased more of the humor fans have come to expect from the series.
Mandarin's Voice
Another comparison worth noting to The Dark Knight Rises is the fact that the main villain has an unusually distinctive voice with an accent of vague origin. Ben Kingsley's slow, methodical delivery is reminiscent of Hugo Weaving. There's little indication so far if Mandarin is actually meant to be Chinese, as he is in the comics. This is in keeping with previous comments that Mandarin is more of a multicultural character who adopts a variety of symbols and clothing.
The Mark VIII Armor
The trailer offered a few glimpses of Tony's Mark VIII armor in action, which is believed to be based on the Extremis armor from the comics. The new armor improves upon the Mark VII that debuted in The Avengers. Whereas that suit could be summoned using Tony's magnetic bracelet, now he can telepathically control each individual component of his new armor. If the new armor follows the example of the Extremis armor, we'll also see Tony gain the ability to directly interface with computer and satellite networks.
The Iron Patriot Armor
Alongside the Mark VIII, the trailer also offered a brief glimpse of the iron Patriot armor. Based on previous news reports and spy photos, we know that Jim Rhodes is the wearer, and that Iron Patriot is a more patriotic replacement design for the War Machine armor.
Sentient Armor
It seems that Tony can't even trust his own armor in the new movie. A brief shot shows the Mark VIII apparently acting of its own accord and attacking Tony and Pepper in the middle of the night. Is the Mandarin taking control of Tony's new toy? Is the film drawing inspiration from the comic book storyline “The Mask in the Iron Man”?
Death of Happy Hogan?
Much of the trailer focuses on the chaos that breaks out as Mandarin orders an assault on Tony's Malibu home. But it's not just the house and the armory that come under fire. Happy Hogan can be seen lying injured among the wreckage. Hogan is once again played by previous director Jon Favreau. Perhaps the decision was made to kill off Hogan now that Favreau is no longer directing the franchise.
New Characters
Aside from Mandarin, the trailer wasn't very forthcoming as far as the new additions to the franchise. The footage included one brief glimpse each of Guy Pearce as Aldritch Killian and Rebecca Hall as Maya Hansen. James Badge Dale as Eric Savin/Coldblood and Ashley Hamilton as Jack Taggert/Firepower weren't seen at all.
Where are S.H.I.E.L.D./The Avengers?
The movie clearly isn't ignoring the events of the Avengers. Tony's opening monologue reveals that he's very much affected by the experience and the exposure it provided to the greater scope of the Marvel Universe. What remains to be seen is how much S.H.I.E.L.D. and the Avengers will factor into the events of Iron Man 3.
As far as the Avengers go, the team effectively disbanded at the end of the movie, and we may eventually learn that the events of Thor: The Dark World and Captain America: The Winter Soldier are happening concurrently with Iron Man 3. In terms of SHIELD, it seems unlikely that Mandarin's actions would escape the notice of a worldwide peacekeeping agency. But currently, no cameos of characters like Nick Fury, Black Widow, or Captain America have been announced.
Mandarin's Tattoo
“Heroes – there is no such thing.”
That's the first lesson Mandarin wants to impress upon his audience during his voice-over in the trailer. However, it seems he may have more of a superhero fetish than he's letting on. The brief shot of Mandarin removing his hood shows that the villain has a tattoo of Captain America's shield on the back of his neck. Is this some sort of homage to the world's first super-soldier, or is it meant to be an ironic display? Is it just a coincidence that Iron Man 2's Whiplash also sported a wide variety of tattoos?
It's even possible that Mandarin himself may be some sort of product of the super-soldier arms race. In the comics, the character is shown to be an elite martial artist in addition to wielding his ten rings of power. By focusing his chi, Mandarin can hit hard enough to shatter even Iron Man's armor. Re-imagining him as some sort of black market Cap knockoff may be a more plausible way of granting Mandarin the same level of fighting prowess. Is this where the Extremis process comes in?