"Eh" seems like a pretty good summary for that trailer. I like a good spy movie, but there's no clear hooks to this beyond "hey Black Widow finally got a movie." I suppose there's also "We get to learn about her past" but Widow's past in the MCU is a bit like Wolverine's past in the comics: it's interesting because it's mysterious, with only just enough info provided in past movies to give you the shadowy contours of her life.
To my mind if you're going to do a spy movie within the MCU you need to include just enough fantastical elements to distinguish yourself from something like Mission Impossible or Jack Reacher. Winter Soldier is a good example of this, being largely a lean and mean spy thriller, but full of jetpacks, super soldier cyborgs, and flying aircraft carriers. Black Widow probably doesn't need quite as much of all that as Captain America, but I've always been of the opinion that a movie focusing on her is an awesome opportunity to make use of Marvel's weird old Soviet characters, of which there are many. Red Guardian's inclusion is a promising sign, but he doesn't appear to be doing anything too fantastical here. I suspect stuff like that is probably in the movie somewhere, we're just not seeing it at the moment.
This may be a result of being an early trailer. Early trailers always have unfinished effects these days, and the biggest CGI shots are likely not ready to showcase. For now, though this wets the whistle. The second trailer will need a little more "oomph."