A semi-geek interpretation of the trailer. (Much speculation, and therefore spoilers, abound):
The first voice-over: Ultron, admonishing the humans that while they want to protect the world, they don't want it to change. He's an AI, and like Skynet and the Borg before him, he's determined that the only way to achieve order is through chaos. So long as humans are in control of the world, it's going to keep being awful. And to make it worse, this dude's got just enough of Tony's personality to manifest his arrogance. This is Ultron's "I am Iron Man," moment.
We get glimpses of bad things. Cap's in shock. Bruce looks like a shell rocking on the floor - Ultron's pushing buttons. He knows their moves and counter moves, and he can't be reasoned with, threatened, or seduced in the way a flesh and blood enemy can.
The "puppet" version of Ultron appears, and everyone's got a Oh $%*& look on their faces -- They killed him, but being that he's Tony's creation, he doesn't die easily, and he's put himself back together. It's no prettier than Tony's cobbled together tin can from the first Iron Man, but it's got him on his feet, and out of "the system" Tony had to control him. But the biggest thing to take away, is that he's now self-aware. The head-crushing of the other robot is sentient, self-built Ultron destroying one of the drones that were his original form. He was basically the next incarnation of Tony's legion of super-suits, and now he's condensed himself into a single body.
The creepy Pinocchio voice over of the singing boy - pure Disney, as P is a Disney film - and it's a beautifully creepy juxtaposition. Ultron is "real" now, and he's a real threat. Here's where we'll see the team fracture, likely in preparation for a "type" of Civil War (licensing restrictions prevent a fully loyal version). Nick Fury's "hopelessly, hilariously, outgunned" speech will come into play, as will the council's fears about the Avengers being allowed out. New people will enter the fray, ushering in "the age of miracles" from the post-credits scene of the last Captain America movie. The gov't will want to lock them down so they can pretend to be in control. Others will say that the world is safer when the "good" guys are free to roam. The war will start over who gets to decide which ones are good.
Enter Pete and Wanda a.k.a. Quicksilver and the Scarlet Witch. (And they'd better be prepared to up the ante from slo-mo Pentagon kitchen scene in Days of Future Past, or their Quicksilver's going to become a nerd pariah.) My guess is that Ultron will "rescue" them from their cages, convince them that he's on the side of those who don't want to be controlled, and get them to help him in his quest to destroy pretty much everything under guise of them seeking vengeance or justice.
They'll figure him out and switch sides, bringing two more Avengers on board, but totally freaking out the government contingent.
Tony's "end of the road" speech likely isn't at the end of the movie. I'd bet it's at the beginning, when they first get a glimpse of the Ultron-bots and things are still seemingly going good.
Someone's upset Wanda, and that is really - really - bad. In the comics, the last time she had a break-down, she depowered like 95% of the supers in the Marvel-verse. No one wants to face a ticked off Scarlet Witch, and I'm guessing that's where the Hulk buster armor comes in. She's using the Hulk as her own personal shock troop of one; Tony has to devise a way to put him down, and rather than kill Banner, I'll go ahead and wager that he sends the jolly green giant into space. (Where he'll meet up with Peter Quill and the rest of the Guardians of the Galaxy in Phase Three.)
In conclusion, the Avengers have unleashed Chaos onto the world. Remember, this is a bridge movie, so things won't end well. (Think of this like the Marvel version of Han Solo going into the carbonite.) They'll win the day, at high cost, gain some allies, but still be broken. They'll be almost at rock bottom, so that once they hit it act three can begin and we'll start the climb through phase three, culminating in the face off with Thanos over the Infinity Gauntlet.
We didn't see the Vision, but given that he's voiced by the same actor who plays JARVIS, I'd also guess that he's built in response to Ultron's awakening and defection. Tony will do what Tony does and that's try to push forward by pure stubbornness, meaning that he'll keep combating one AI with another until he gets it right.
And that is why the geeks are freaking out. Marvel's about to light the fuse on a universe-wide explosion that will feed into the remaining franchises from here on out.