Paul Graham makes an important point with this observation. Given the YC mantra of "building what people want", it isn't enough to have domain expertise and insights into what people want. You still need the skill set in the founding team to build it.
One of other four original YC partners, Jessica Livingston, provided similar insights in her advice that a founder needs to be good enough at programming to "judge and collaborate with potential cofounders, and perhaps to build a prototype":
Through my work at Y Combinator, I’ve observed a lot of patterns among early stage startups. With female founders, I’ve seen a lot of cases where the woman is an ass-kicking domain expert in some field and has a clever idea but she’s completely hamstrung because she can’t program.
Specifically, she can’t build the first version of her product and is forced to find a cofounder who can. Because she can't judge technical ability, she'll often choose the wrong person for the job.
While searching for that quote, I came across some other great links to Jessica's insights. Those links are so valuable that I'm tempted to change this commentary to something originating with Jessica instead of Paul. Maybe next time!
Here is a preview:
All too often, I’ve seen founders build some initially mediocre product, announce it to the world, find that users never show up, and not know what to do next. As well as not getting any users, the startup never gets the feedback it needs to improve the product.
So why wouldn’t all founders start by engaging with users individually? Because it’s hard and demoralizing. Sales gives you a kind of harsh feedback that “marketing” doesn’t. You try to convince someone to use what you’ve built, and they won’t. These conversations are painful, but necessary.
Also, who knew that Barry Manilow's drummer was the customer that drove the key change to Airbnb's business model?
Jessica Livingston on "Why I Started the Summer Hackers Program"
Jessica Livingston's comprehensive list of advice to startup founders: Female Founders Conference Keynote
Jessica Livingston: Why Startups Need to Focus on Sales, Not Marketing