Note: this review builds on Stephanie Zachareks' review of the movie at movieline.com
Not to say Loki doesn't have presence, but the story proved to be about the team settling together in a very satisfying fashion, with everything else but interjections to this realization.
Personally, the agent getting tongue-tied before an American legend, perhaps because it was presaged by his being highlighted, playfully, certainly not sincerely, but still a bit oddly as a romantic rival to Stark, kind of felt like he was being set up for something -- the overreaching kid, due a sad fate for being likened to things he'll never be -- a manly hero; a playboy -- and for never being able to subsequently quite pull himself tightly together again thereafter. But you let that momentary consideration dissolve because its purpose of helping limit the sense of Captain America as anachronistic, now irrelevant -- a joke -- that nobody too much wants while we're all becoming entranced to bond with Eastwood, old American virtues, locales and industry, is effectively lessened here. Anyhow, the moment felt too functional and too much like it was spotting one guy out to be affecting. To me, the moments that worked best were when the heroes have had their fair estimation of one another and have begun to settle in. Thor standing alone quietly with Hulk for the first time as mates -- which, other than Hulk's Indiana Jones-like trouncing of the gloating, over-estimating opponent, might be the film's most satisfying moment -- worked for this reason first, and then secondly for the terrific follow-up humor it spawned.
I'm not sure Black Widow's best moment was the one you mentioned. I was more struck with her encounter with Loki, another plausible instance where something human -- there's just no denying it! -- could catch short even the likes of "magic and gods"; and so much more satisfying than when an F-18 or a nuke are used to do the same. It was a terrific surprise; Loki blanched and guffawed, as we did. And we were pleased that Whedon didn't let a talent she'd after all shown she had simply surrender itself because before someone ostensibly way out of her countenancing; and I thought good for you Whedon for giving something pronounced to the more simply human characters -- with Captain America of course getting undaunted leadership, as well as emblemmanship of the times -- to help settle them in experientially amongst their powerhouse teammates as legitimate peers. Hawkeye didn't get a standout trait; but they did make his arrows something sorta akin to Iron Man's arsenal. And with him more or less the one exception, and with him being played by a movie star actor, and with us wanting one or two of the Avengers to be allowed to sulk a bit in the shadows -- good enough.