Suggesting that startups are inherently interesting misses the fact that a great many of them are rehashes of previous successful startups, which makes one startup feel just like that other startup you just left.
The sense of ownership is also highly variable. It depends on how much of a control freak the technical founder is.
Personally, I choose startups because of the high tolerance most startups have for individuality. I work from home in the mornings, have a standing desk at work and take walks in the middle of the day without anybody freaking out.
Thus, my risk is much lower than a founder's.
Your expected opportunity cost in the case of failure is smaller--like you say, you draw salary as soon as you start (though the salary is often less than market rate). I will point out that employees are usually less well-informed than founders about a company tanking, so will usually have a couple of months lag in looking for another job, and that "I was employee 3 at Vunge Games" doesn't sound nearly as good as "I founded Vunge Games" when you're at interviews.
Your expected payoff in the case of success is probably at least an order of magnitude smaller, and from anecdotal evidence seems to not be much more than it takes to make up the market wage shortfall over the course of your employment.
This isn't intended to be pedantry--I figure that all of them (but especially the expected opportunity cost) are likely to vary based on region and are worth noting as hidden costs. But from my perspective, being an early employee at a start-up is a bit of a mug's game when the alternative is bootstrapped founding from corporate employment.
But it's not about getting "utterly screwed". It's about making a rational decision that some things in life are worth more than financial reward.
Working at a startup has a multitude of non-financial benefits. You'll work with the latest new technologies. You'll play a key role in a small team. You'll be forced to learn all kinds of new skills. You'll meet lots of people. You'll acquire a sense of ownership and confidence that you may never feel when working in a large corp.
If you've got a family to raise and you're worried about your retirement savings, by all means, follow the math and don't expose yourself to the near-certain financial ruin of a startup.
But if you're young, adventurous, or simply value interesting work over a high salary, it can be a lot of fun to get utterly screwed with your measly 2%. I enjoyed my last shot.