Just here to chime in and give my poke to the beaten to death horse that is BruTasha. I personally didn't have a problem with their relationship; I personally didn't think their relationship was ever a love relationship. By that I mean, there was more of a flirtation element and along the lines of wanting to connect with someone who was a kindred spirit; it was more about that longing for connection than the connection itself. I believe Joss used this more as a means to exploring their internal issues more than giving the movie a love story element.
Using this faux-love story, we find that Natasha has a haunted past and truly wants to find some semblance of normality and feels like she can find that in someone who also seeks the same thing; she values a man who doesn't value his power because she's always been around men and woman (including herself) who've abused it and her tragic past is just an indication of that. We also find, though most fans who've watched TIH would know, that Bruce also seeks some semblance of normality and will never find it because he's haunted by his inner green demon; he also can't be with anyone he loves because if he truly loved them, he knows he'd only do them harm.
This is why I think Joss chose Bruce/Natasha over Clint/Natasha; we get far more character seep out of the interaction of Bruce and Natasha that really lends to their development and character exploration of each of them in Age of Ultron. I don't think it'll be a love interest that'll be a through-line in the MCU.