There are a lot of very thoughtful, insightful posts already on this thread. One thing that has not been stressed enough is that a sub actually gets pleasure from being dominated, whether that is through pain, restraint, orgasm control or any other aspect. It is vitally important to understand the psychology of the sub and that does indeed take time. I also think that intelligence, communication and the ability to articulate are paramount.
The s/D power exchange has nothing to do with sadism or abuse, but more to do with trust and care. It takes an awful lot for a sub to give control of their own pleasure to a dom who knows what they are doing.
The last thing I will add here is that there is little a sub dislikes and is frustrated by more than a dom who does not know what s/he is doing.