Inconsequential writing. Weak motives. Boring villain.
Lacing decent but choppy fight choreography with a theme of family forgiveness and jumping between international locales doesn't make a spy film. Utilizing pseudo science tech without previewing the tech earlier in the movie comes off as lazy writing instead of a payoff.
There were plenty of opportunities to add depth to Romanov, her motives, her personal drive. Instead, when questioned directly about why she joined the Avengers, she handwaves a reason, "because family."
Acting is great. While Harbour and Weisz are both stuck in 1 dimensional characters, they give the characters the appropriate bravado, charisma, and hubris.
There is good sisterly chemistry between Johansson and Pugh, but the lack of story telling precludes the audience from relating to the characters. Bookending the movie with throw away lines from the prelude as the girl's are young is a cheap emotional trick that wears off quickly.
Cinematography was good. Action is easy to follow, minimum use of shaky cam. But the writer and director seem to forget that Romanoff isn't a super hero when taking hits while fighting, crashing, falling out of an airplane, with nary a bruise nor scrape.
The Natasha character has the potential of portraying significant depth. Established MCU relationships convey that the other members deeply trust and respect her. She is allowed to be a real person, with ideals, faults, and expectations of others. This movie instead compresses her character down to a plot device. An excuse for others to reminisce about glory days.
In particular the story should have done more to connect Yelena to Natasha before Yelena reached out (and sent a bunch of the priceless antidote in the mail???) There was no motive as to why Yelena would seek out Natasha, and the script glosses over dialogue that could have fleshed this out.
If instead there was story showing how the two developed a strong bond prior to Romanov escaping, the rest of the movie would have had significantly more impact. This could have raised the stakes by allowing the audience to care more about Yelena and the other widows, and could have built to a beautiful reflection of Romanov's own sacrifice that we already know will happen.