-Fast-paced start-up environment can sometimes be a little chaotic
-Successful start-up means owners & management are all fairly new to the game of running a business - the "fly by the seat of your pants and learn as you go" style sometimes leads to mistakes. That being said, good decisions are made more often than bad and mistakes are learned from.
-Rules/standards are sometimes inconsistent and enforced inconsistently
-High pressure job - start ups can't afford "dead weight." If you're not doing enough to help the company succeed, you'll likely be shown the door. I don't mind pressure - that's when I'm at my best - but if you can't handle working on a hot seat, this might not be the company for you.