A slightly less cheerful view of these things. Note that this is coming from the view of being an early employee instead of a cofounder (been there, and the calculus is totally different). Note also that this is just in my experience...mileage in a place with a better startup culture or just better teams may vary. Note lastly that this is from a technical slant--biz folks may get something different.

1. You are directly tied to the success or failure of the company

Translation You aren't going to get rewarded for your hard work--the "visionary" cofounders will, probably--but you are uniquely positioned to ruin everything for everyone! So don't question how things are going, don't slack off or take personal time, and make sure that you milk your personal connections for all they're worth to help your new company...even if they leave you looking like a jackass during, say, recruiting referrals.

2. Being in a startup gets you direct access to talented people that can mentor you and help advance your career path

The very same people already available in your community and online! In fact, odds are that the cofounders are super "talented", especially as experienced developers and engineers--otherwise, why would they have been able to raise funding? Make sure you mimic everything these talented people do: skip documentation, don't bother with long design cycles, respond to every customer feedback with massive overnight code changes because agile, or feel free to ignore customer feedback because visionary.

3. You must be able to adapt quickly

Remember, corporations are stable, and stable is just another word for slow and boring. Being at a startup means being able to adapt to changing circumstances quickly, whether it's fixing yesterday's mistake or this morning's. If you spend a long time--more than an hour--planning major business decisions or technical changes, you are moving too slowly and are basically just wasting time. We're trying to kill it, so what the heck is your holdup?

4. You have more flexibility to be creative and spread your wings

Structure, like regular paychecks or company policies, is really just there to prevent creative people from solving their problems creatively, so we'll ignore them. Now, this isn't to say things are an anarchy--far from it! There's a solid informal system of checks, balances, and surveillance used to make sure that everybody is on the right path and fitting in and spreading their wings to the best that they can. You can try anything you want, as long as your wings don't ruffle the feathers of the founders!

5. Being a self-starter is a necessity, not just a bullet point on a job description

You've got to be able to spot opportunity and seize it--the founders are too busy doing "business development", fundraising, and "customer acquisition". So, naturally, some things are going to slip by. Things like project management. Or recruiting. Or architecture. Or 3rd party developer support. Or infrastructure. Or tooling and code review. Or sales. Help them help you help them!

6. Be prepared to wear multiple hats and be a jack of all trades

You are on a small team, so of course you need to do many jobs. We need full-stack developers: people who are comfortable in C, Javascript, Linux systems administration, LDAP, Kerberos, Docker provisioning, network technologies, UI/UX, and SEO. Now, in reward of your lifetime of learning, we're not going to let you work on that during normal hours and especially not without claiming it as company IP by default--that'd basically be hurting yourself, because that could hurt the company. Oh, and can you come in on Saturday? One of the founders needs help with git.

7. Your team is everything

Look, products fail all the time, and the market is hard. Fact is, your company might not be able to maintain traction. So, eventually, there needs to be an exit--and that's not going to happen unless an acquirer sees a happy, cohesive team of engineers and growth hackers. Remember that every time you have a disagreement about how things are being done, or have an issue with a teammate: you don't want to screw up an exit, do you--why, that'd ruin all the hard work you've been putting in the past few years.

~

Startups are amazing life experiences when they're good, but when they're bad, it's worse than sex with virgins.

In the latter case, nobody knows what they're doing but at least you know who's getting fucked and when.