(b0ttler0cket):
I think this question has a lot to do with risk and reward. Graham addresses this (at this point, it's a bit of a blur. Whenever I hear something like this, I just randomly pull out some of this reasoning).
I may not be able to paraphrase exactly, but in regards to your question about regret and risk, the reasoning may have gone something like this:
1. Age is important. Younger people can afford to take more risks for the simple reason that they seem to have more time to mitigate any damage from the mistakes should they fail. Also, younger people are less likely to be parents already or have a family.
2. Attitude is important too. Some of this comes down to what you can better tolerate. Startups are like sprints. Employment is like long distance running. Do you want to cram the bulk of your expendable energy into a few years of intensely hard work (and stress) with an uncertain payoff, or do you want to work moderately with a very decent payoff and still have the time to enjoy family, a relationship, long walks in the park, jogging after work for as long as you want, weekend-long trips to the beach. I feel like a lot of people in HN, if given the choice between startup and employment, would say startup hands-down. But if you look at it this way, I believe the decision is a little harder for many people.
3. Maybe the most important. You have to believe in yourself, your product, and your ability to come up with ideas if you want to do this. I cannot picture a scarier situation (in regards to startups) than starting one full of doubt, anxiety, and overall insecurity. If you do this, you have put all your chips in the pot, keeping in mind that there's no bluffing and no turning back (well there might be turning back, but you definitely shouldn't be planning to). You have to have the cards to back up your all-in. Having the cards means having the guts to stick it through, having some level of smart, being adept at troubleshooting problems, and having a fairly good idea from the start even though it's subject to change in various degrees.
I believe (1-3) addresses your concerns about regret to commission of a startup. In terms of whether a startup makes you unemployable, I do not see why a startup would bar you from further employment. Potential employers could point to the fact that there may be a hole in your CV where you weren't employed for a year or two, but you can easily describe your startup experiences and couple that explanation with why that experience makes you more valuable as an employee. Remember, you still have to be able to speak on your good points with ease and decisiveness whether or not you've engaged in a startup.