My wife is terrified of heights but loves this ride. In my wife's case, she is very uncomfortable standing next to a balcony railing just a few stories up, but if she steps away from the rail she is fine looking out. Her and I believe most people's fear of heights comes from looking down from that height. On ToT you never look down, only out.
As far as darkness, if the dark rides like Space Mountain, Splash Mountain, and Dinosaur both someone then Tower of Terror will probably also bother then. There are moments when it is completely black, but they don't usually last more than a couple seconds or so.
For a lot of people who are not fan's of ToT, it is the combination of elements that they don't like. It may be my wife's favorite ride, but my daughter (13) hates it primarily due to the drops (and I really think the anticipation).