I haven't played in long enough that this thread actually brings back a lot. I'm not unaccustomed to the feeling of power and responsibility in daily life, nor being the center of attention. So I don't particularly notice those things during a scene... they're just not "different" from what I'm often feeling anyway.
A lot of what I do feel though is dependent on what we're doing. For instance if we were to go out and she had a plug in her ass, what I'm feeling most of the time is a sense of amusement. I think that's true most of the time I'm acting as a Dom not in an actual setting. Seeing the reaction to commands, or watching her squirm for no apparent reason, and knowing something nobody else in the room knows... and most would be shocked by. That for me is both insanely arousing and amusing.
Most of the time I feel a deep affection for my sub, and that plays into the mixture of fear and... what Jenn said, it being all about her. Despite the attitude that she's "there for my pleasure", really, nothing is more important to me than that she enjoys the experience. I want her to like it, I want her to ask for more, and to want to do it again, and I want her to be satisfied at the end of it.
My biggest fears are something going wrong and hurting her, and at the end of it hearing something like "Well, that wasn't very good." The last is probably closely related to the same sexual fear almost every guy has though, I imagine.
Also, during after care... I get this incredibly sense of emotional peace. Like I've broken down some of the emotional support in my sub, and there's this need emanating from them for additional support and love. It's really comforting to me to be that support after a scene, it makes me calm and incredibly happy. I think it allays my personal vanilla insecurities, which seem to disappear in a scene, and for a while afterwards.