It's a startup, which isn't for everyone. Since it's a relatively small team, everything you do (or don't do) will impact the company. It's not a place for people who like to blend in with a larger team - you'll be expected to think creatively about your role, come up with new ideas and generally play a part in moving the company forward. You'll also need to be adaptable because things can change a lot, and quickly, with regard to products/direction/etc.