The most exciting thing for me is how people react to the product that you have build. Even if the feedback is bad or good it is feedback and it so valuable to know if you're on the right path. The scariest thing is if nobody comment your project, then you are wondering if this is even worth the time that you spent developing it.
I don't think there's anything to be scared about the product launch, We are launching something which doesn't exist anyway :) but yea if we do not improve the product with time and feedbacks, that would scare me the most.
And exciting thing would be planning a strategy to capture the market as soon as possible, with reasonable investment of-course.