The B/D S/M scene is pretty diverse. There are players who are pain sluts and that could be a serious psychological issue. And there are people who are power hungry who can prey on them but most of the people are into a form of "power exchange" which is often an antidote for their life. Often powerful people in their daily life need power taken away in their sexual life and fantasies. The reverse is also true. It tends to center them emotionally and psychologically.
While part of the scene is decidedly visual, one main component is "it's not what it looks like, it's what it feels like". Anyone who has ever had a "submissive high" or been in "subspace" will tell you that it blows the doors off vanilla sex any time. The most rewarding of the experiences is with a loving partner and the mental "bonding" that occurs because of the intimacy involved can take couples to a whole new level of trust and love.
There are also people are into the whole costume part of the game and these can also be seen in other examples of life like civil war re enactors or Renaissance Faire devotees.
As for the pain aspect of S/M when things get going the endorphin high is so intense that the whole pain/pleasure thing gets mixed up and people are flying anyway. Obviously that can't be gotten out of control that's why players play within a trusting group.
For any more information come on over and I'll show you. If you've been bad, watch out.
Just kidding. I read all this stuff in a book once.