In the beginning it was a sharp erotic thrill at being so helpless within my husband's power, and an intense desire for immediate marital relations. Now, it's more of a warm satisfaction at making him happy.
Also fear at the beginning. I knew he was a good man, or I wouldn't have married him (duh!), but I didn't know what he would do with the power I was giving him.
As Abraham Lincoln said, "Nearly all men can stand adversity, but if you want to test a man's character, give him power." Well, as I gave him power over me, I learned about his character, about how much he cherished me and valued my safety, and how nice he WANTED to be to me when he didn't have to be nice to me to get whatever he wanted from me (in or out of the bedroom).
For many years now I have trusted him completely, so I feel controlled, protected, loved, and cared for.
This is wonderful, but it does have one strange and unexpected disadvantage. He can't scare me any more! Even when he tries, I know that even if he does things I don't like he will make sure that I am safe and able to handle it, so a certain excitement that came from fear is long gone, but I'm okay with that.