The second trailer for The Avengers 2: Age of Ultron was just a cavalcade of new info surrounding Marvel's next money-printing machine, so missing a few things is pretty forgivable.
There were three huge bombshells in particular that Jim Galizia already pointed out on this site, but yet again, Scarlett Johansson's Black Widow got the shaft on reveals.
Here's the trailer again in case you missed it or just want to watch it for the tenth time:
At minute 0:39 of The Avengers 2 trailer, we see a younger version of Black Widow being restrained and experimented on:
WARNING: MILD SPOILERS FROM CAPTAIN AMERICA: THE WINTER SOLDIER ARE BELOW!
One of the more overlooked aspects of [Captain America: The Winter Soldier](movie:254973) was the age and hidden backstory of Black Widow as it relates to Hydra.
For example, two specific moments in The Avengers hint at hidden secrets surrounding who she really is. The first being the line, "I know what it feels like to be unmade!"
The second being the debt she owes Hawkeye.
In the comics, [Black Widow](movie:1070824) was born in 1928 and trained by Russian spies. The Soviets "enhance" her physically and mentally, prolonging her lifespan and making it possible for her to still be kicking ass with Bucky and Spider-Man in 2014.
But the movies don't address any of that. We just get the bad-spy-turned-good spy story only casual comic book fans would know about the character. But it's Marvel. They've probably been planning something way more elaborate and compelling for Black Widow's story without being identical to the comics.
I believe that Black Widow was at one point given a derivative of the same serum that transformed Steve Rogers into the Captain of America. That would make her a puppet of Hydra/Shield, before they were discovered to be one and the same.
And of course, Hawkeye probably played a part in saving her from these guys and convincing S.H.I.E.L.D. to take her in. Hence the whole life debt thing.
Now I'm not sold on the idea that she's secretly 80 years old but only looks young, only because The Winter Soldier goes to big lengths to dispel that theory by revealing she was born in 1984 (which his probably a clever transposition of when she was experimented on in 1948. That sure worked out!)
But I'm all for the idea that she was once a Hydra experiment made to mimic the legendary Captain America, and it's clear Avengers 2 will finally bring that story to light, though a solo film would've been much more fun, if you ask me.