I'm not a lean startup zealot, but there are obviously lots of aspects of it that can lead to a ton of insights about your business. Talking to customers is, of course, a good thing. I'd never argue against it.
I have seen, however, the customer development process wind up looking like a street corner salesman selling watches out of the inside of his coat. "Don't what this?" "What about this?" "I've got more in the car of my truck if you want them in blue."
What's lacking is an innate understanding of the customers problems before they go through the ideation phase. I find that some of the most sound entrepreneurial efforts are one where the founder has lived the problem uniquely in some way. Either they actually were the customer (and by customer I mean someone who pays for this kind of service) or they've literally spent years thinking about it--as an enthusiast or insider. I didn't come from the recruiting industry when I started Path 101, but I had been obsessed with career discovery as a mentor and educator for eight years before I thought of an idea I wanted to pursue.
That's different than, "hasn't this happened to you?" kind of approaches. Just because you had to wait on a line once doesn't mean you know nearly as much as the person in charge of people flow and traffic at Madison Square Garden. Similarly just because you've been to a conference doesn't mean you know nearly enough to provide the industry with a game changing piece of software to run them all. Its possible, but unlikely.
If you don't have unique insight into the nature or the problem, customer development is going to be random and unproductive. Its like being a doctor where you don't know why the patient is sick, but they feel better after you give them a Tylenol. It may appear like you've solved the problem, but it was just dumb luck that you addressed a symptom. That won't do much in terms of informing you as to a long term course of treatment or what happens when your Tylenol doesn't solve the problem as well as you thought it did.
I think a lot of companies suffer this when they're in new entrepreneurial ecosystems or ones outside of major cities where innovation is happening in the biggest industries. The startups tend to be really consumery spins on things they've seen on Techcrunch--as opposed to more organic solutions to problems they've experienced uniquely.
The first time I ever met Chantel Waterbury from Chloe & Isabel, she told me in detail about the issues in the direct sales jewelry business--a huge market but one that suffers from inferior product quality and poor branding. People don't exactly associate "fashion forward" with "tupperware for jewelry". To have enough insight into how to solve that, you need to be more than just someone who likes jewelry. You need direct experience to understand how to get high quality and stylish fashion jewelry made at a reasonable price. She launched fashion jewelry for major brands and was the top seller of CutCo knives on the West Coast--paid her way through college doing it. Sure, she'll pay attention to her customers, but she's starting from a position of experience that at least puts her in the right ballpark day one. That's what makes customer discovery a place to hone your idea with relevant feedback and not a random spin of the Wheel of Startups.