Though Ebert gives out four stars like candy, these two paragraphs in his review sparked some interest in me seeing the film.

But the film doesn't depict the terrorists as villains. It has no need to. Like everyone else in the movie they are people of ordinary appearance, going about their business. "United 93" is incomparably more powerful because it depicts all of its characters as people trapped in an exorable progress toward tragedy. The movie contains no politics. No theory. No personal chit-chat. No patriotic speeches. We never see the big picture.
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There has been much discussion of the movie's trailer, and no wonder. It pieces together moments from "United 93" to make it seem more conventional, more like a thriller. Dialogue that seems absolutely realistic in context sounds, in the trailer, like sound bites and punch lines. To watch the trailer is to sense the movie that Greengrass did not make. To watch "United 93" is to be confronted with the grim chaotic reality of that autumn day in 2001. The movie is deeply disturbing, and some people may have to leave the theater. But it would have been much more disturbing if Greengrass had made it in a conventional way. He does not exploit, he draws no conclusions, he points no fingers, he avoids "human interest" and "personal dramas" and just simply watches. The movie's point of view reminds me of the angels in "Wings of Desire." They see what people do and they are saddened, but they cannot intervene.