In my case I think that apprehension is a much more accurate term than fear now that my sub and I have learned where the limits are. My sub doesn't "fear" me bacause she knows that I know exactly where her limits are and that I'm going to push her to them but never beyond them. In the moment I suppose that there is always going to be a bit of nerves or apprehension but I don't think it crosses over into real fear anymore. That being said I can see how for a sub thats still figuring out where their limits are it could be frightening to explore.
I was a little afraid when first starting out in my role as a Dom. I didn't want to push her too far too quickly or expose her to something she wasn't ready for yet. IMO one of the most detrimental things a Dom can do is push the sub past their limits. I will say that the fear I felt did excite me and the excitement I felt was palpable. The more I pushed the stronger it became. I'd say that the fear that we both felt was absolutely necessary for the trust between she and I to grow. It may sound oxymoronic but without the fear of pushing too far or being pushed too far I may not have pushed far enough and she may not have ever known exactly where her limits are.