You will learn a lot of things but the wrong way.

It’s true in a very small business, you will have the opportunity to learn more than in a large organization. But you will be probably acquiring those new skills without structure. It’s cool to set-up a build system for your start-up but there’s a big difference between setting up one for a couple of programmers than for a large team on a critical production path within a structured production environment.

Here comes the power trip!
If you think you will avoid tyrannically vertical management by working at a startup, think again. You will soon discover that most founders go into a power trip mode, especially when they get their first employees. You will find yourself handling inexperienced self-made “managers” that have very little training or actual business experience.

No time to fail!
Because of a lack of experienced management in most startups, they will not give themselves any head room to fail. They will pursue every opportunity with the equal amount of effort & resources. This means that if you work for them, you will be in a constant state of “crunch”. You will not have the time to do any postmortems after a milestone or delivery. You will be in a constant state of production "rush". This will not let you have the flexibility to fail, learn or reasonably re-adjust deadlines for your next deliveries. You will just persist any bad behaviors or decision making throughout the production until you reach the critical point of failure. Any type of success, will be the result of guesswork and not because of well-thought-out decisions or a solid production process.

They think they cool!
A lot of young startup founders have a tendency to be arrogant, believing that they the next “big think”. Even if this attitude might work in seducing naive investors, this is poisonous when dealing with employees. They will have a hard time with listening to any critical commentary from they own team and by consequence, will persist with that attitude until they burn-out all they resources.

They call themselves a “startup”.
The number one reason you should avoid working for them, being a “start-up” is a transitional state. The minute you have employees, customers & partners, you are not a “start-up” anymore but a small growing business with social & economic responsibilities. If they want to stay a “start-up” and embrace fully that culture, then they should move back into they parent’s garage.

They bubble is about to burst!
As we are witnessing in recent trends in the market, investors are wisely starting to pull out of the "false" startup economy which is mostly driven by the irrational fear of missing the next “big thing”. Governments, established business & investors are starting to see thru the flood of “one-trick pony” incubated "venture capital grabbers". The backlash against this new tech bubble is coming and will probably wipe out most startups off the map, so working for them is now a high-risk proposal with very little chance of return of investment, especially for those “shares” they promise you in exchange of your "free" overtime.

I now walk away from anyone that describes they business as a "startup" because it's like an adult describing himself as being a "teen". Why would I work with a company that fundamentally embraces an "adolescent" state? Many people can innovate, many more can create but very few can organize and build a sustainable business.

And those are the people I want to work with, those who can innovate but also run a solid business. But taken all this into account, there's some quality small innovation-focused companies in the current "brouhaha" of startups, we just need to start to "filter them" well as investors but also as potential employees & partners.