The biggest thing to keep in mind is that you will be transitioning into a world of "butt time". Your employer will care far less about what you do than where you do it. You could institute world peace and make your company a billion dollars from your arm chair at home and your boss will still call it vacation.

As a bit of advice for working in this world... You have no control over when you work, so you must regulate the density of your work in order to keep from burning out. What you would do in a single day at your startup takes people at "normal companies" many days or even weeks to accomplish.

Also, guard your time off like For Knox. If you are willing to "sacrifice for the team" your boss will gladly steal as much time from you as you are not willing to defend. Work ends at 5, anything further will need further compensation, be it overtime or extra vacation. Your boss asking you to work extra with no extra reward is stealing, pure and simple. Use more tact to say that to your boss, but make no mistake, this is what it is.

Lastly, try not to think too much. 90% of what goes on in an office is inane nonsense having nothing to do with the bussiness at hand. Its just part of the deal. Dwelling on this will only frustrate you. Get your zen on and roll with it.

Good luck.