I watched the first installment of HBO's "Angels in America" last night. The opening scene (as the New Yorker noted) is wonderful; the camera dances in and out of the clouds as it moves across the country from San Francisco to St. Louis to Chicago and finally to New York City, where it settles in Central Park. From there, though, it's very uneven.
The acting ranges from great (Meryl Streep's characters) to the banal (Al Pacino as Roy Cohn, who makes the same "If you want to do good, you must embrace evil!" speeches that he's been making since "The Godfather"). The cinematic aspects likewise range from beautiful (the opening scene) to terrible (dozens of hoaky moving statues).
The biggest problem, though, is that the dialogue from the play just doesn't translate all that well to TV. TV is almost entirely a visual medium; the audio is almost inconsequential. The theatre, however, is a much more broadly sensory experience, since you're watching actual people and not just passively absorbing images from a box. So the dialogue matters much more in the theater.
Moreover, in the theater, the dialogue almost automatically takes on a symbolic aspect to it, whereas on television we generally want realism and/or action. (COPS and "The Simple Life" are the ultimate expressions of this – one is the seedier side of real life, the other is carefully orchestrated to appear as if it's real life when in fact it's more like a psych experiment).
The upshot is that the characters still talk like they're theater characters, with heavy-handed discourses about death, long-winded philosophical arguments, etc. which move things along in the play. But television viewers expect the action to move things, not the words, so it ends up feeling slow and self-indulgent.