Hopefully, you are working full-time if you go to an incubator :)
Assuming this question is about work-style not risk-reward, I have the following insights:
- The biggest change is that you are in charge of your day, not your manager. It is very easy to get distract by the glamour of meetups, TechCrunch, and posting on HackerNews (love that Karma). There is no overlord telling you where to go or what to do. It is you and your company (or idea). Even if you have a CEO and you are the first hire, most likely the founders are going to be busy. They expect you to be an awesome self-starter.
- Work life balance no longer applies. In a startup, your identity is wrapped deep into your company. Your work is your life. Startups require a significant amount of your time, but a TON of emotional investment. Prepare your loved ones for the bumpy road ahead.
- Most things suck, but a few things are good. Startups prioritize. There are going to be things about your product that suck and things about your product that are great. This metaphor also applies to the entire business. A lot of what you will be doing on a day-to-day basis will end up adding little value to your company. You are trying to figure out your bread and butter but you will lose a lot of loaves and cream along the way.