For those that don't know, I'll spoil the bit everyone is talking about:
After Bruce Banner / the Hulk is manipulated by the Scarlet Witch into going on a rampage in a Wakandan city, the Avengers team limps back to the (until now) unknown home of Hawkeye's family. In a pretty revealing scene, Black Widow confesses that she wants to "run with it" by fleeing with Banner from the life of an Avenger. Banner rejects her advances because he believes that she wants the life Hawkeye has created in secret, and that is something that he physically can't have both because of his genetic makeup, which would make procreation deadly (I assume), and because he believes the Hulk will be a threat no matter where they go. Black Widow responds by telling Banner that she had been sterilized in the Red Room as part of her graduation ceremony in order to make her a more efficient assassin; Banner, she says, isn't the only monster on the team.I'm not going to spend a lot of time interpreting this scene, but I will say that I think the scene is poorly written, which leads a lot of people to assume that Black Widow identifies herself as a monster more because of her physical "defect" than because of who she was in the past. It's hard to argue with this interpretation given the way the scene frames the "monster" line, and that's unfortunate when you consider the effort the MCU has gone to subtly suggest who Black Widow used to be before she joined S.H.I.E.L.D. -- the line about her ledger of red is particularly important here.
Personally, I interpreted the scene both ways. Given the context provided by Black Widow's appearance throughout the MCU, I'm inclined to think that the scene is meant to connect Widow's monstrosity to the person she became as a result of the Red Room rather than a monster strictly because she cannot procreate. That makes a good deal of sense when you think about who she had to become -- an efficient murder who, we can only guess, has killed a hell of a lot of people, including innocents -- and why it is significant that her and Banner are the ones to have a romance: they're the members of the team who are closest to understanding one another. However, I also understand why the scene is read as focusing on Widow's reproductive abilities, since the line about being "a monster" is tied into a conversation about procreation and monstrosity. Banner, after all, heavily implies that his monstrosity is tied to his inability to have a domestic life; he even forecloses the possibility of a variation where he is not a biological father because he believes there's nothing he can do to remain stable as long as the Hulk can be brought out. So Widow's response to Banner is an attempt to explain why they are alike, and part of that almost mandates her to correct Banner's "I can't have family" fears with her own version of the same.
Black Widow has been a part of the MCU since Iron Man 2 (2010). When she returned in Avengers (2012), she had ceased to be the briefly-onscreen-badass spy from Iron Man 2 to one of the major components of the Avengers team. After all, it's hard to imagine the Avengers without her, which is a distinction that can't be said about Nick Fury in Avengers: Age of Ultron (2015). Then she appeared in Captain America: The Winter Soldier (2014); there, she became more than just a side character who kicks butt: she was literally Captain America's co-conspirator/co-warrior/co-character. If you removed Black Widow from Cap2, the narrative completely falls apart. She wasn't just significant, but integral. And that's an important thing to note, because by the time we get to Avengers: Age of Ultron, there's a clear progression for Black Widow (more so, I would argue, than any other member of the Avengers team): she has literally gone from a side character who briefly fights on screen to one of the most important characters in the entire franchise. An Avengers: Age of Ultron without Black Widow is almost like a Captain America: The Winter Soldier without either Captain America or the Winter Soldier. It just doesn't make sense.
severely cut one of its MC's storyline down to an almost nonsensical splatter of scenes in order to set up the next Avengers story and in order to save space.
Avengers: Age of Ultron isn't Black Widow's story. We might want it to be, because some of us really love the MCU Black Widow. But it's not. It's a story about how these characters interact as a team and how their internal and external characters and conflicts affect a team environment. All these elements make so much sense for Captain American, Iron Man, and Thor because we've already seen their stories. We don't have to go into Iron Man's long back story because we already know it -- the same for Cap and Thor. But we don't know Black Widow's story. It's absent. That the film tries to fill in the gaps only reveals how much we don't know and how little we have with which to place Black Widow alongside Iron Man, Captain America, and Thor as fully realized characters with clear and fully established motivations. Hell, even Bruce Banner has had his own story told, one of which is technically part of the current "canon." Even he has the benefit of not being "gapped."
The most obvious reason seems to be that Marvel doesn't believe in Black Widow. When it comes to representing her character in the toy market, she is non-existent, even when the toy refers to a scene specific to her character. Even the CEO of Marvel Entertainment, Ike Perlmutter, doesn't seem to believe in female-led superhero movies, which makes one wonder how much control he will have when Captain Marvel comes along (hopefully, none).
But it's obvious that Marvel needs to make it happen. A Jessica Jones series on Netflix isn't enough. A Captain Marvel movie in 2018 isn't enough. What we need -- and have always needed -- is a Black Widow movie. If not because it will fill the gaps in the MCU or because the character deserves better treatment, then because action movies with female leads can do remarkably well and because young women deserve to see more heroes like themselves on the big screen.