Don't bring out the torches and pitchforks, but I may inadvertently (okay, maybe not so inadvertently) dispel the power of the legend. If you want to understand why all the hype, read on. If not, just know that I am not bashing on the success, but rather explaining the science behind a phenomenon.
For a number of years I drove past two locations of In-N-Out Burger morning and night. I have eaten at several locations, a number of times. The food is fresh, inexpensive, and served quickly. The locations are clean, attractive, and usually energetic. The same can also be said for many other fast-food restaurants across the world.
Looking at the quality claims made by In-N-Out reveal their use of 100% pure, high-quality beef. No fillers, additives, and preservatives. Is this much different from the competition? You may think so, but it isn't. In fact, there's another chain who does the same which may surprise you. I am not saying there is no difference between In-N-Out & the world's most famous clown, but there are many things the same.
So why does an In-N-Out grand opening result in people camping out? Excellent branding. They always have been privately owned, which allows them control of the brand, quality, and experience which their customers enjoy. The company also maintained their expansion within close proximity, most locations in California, and they have slowly spread to adjoining states where they could control the supply chain and properly manage each location. A great management strategy maintains quality, which extends a positive experience with the brand.
There are also several factors which affect the psychological connection with the legendary chain. Beginning in southern California, they had access to a large number of vacationers. People on vacation are pumped full of endorphins (technical way of saying they feel good) and that results in general enjoyment of most things, even waiting in lines at Disneyland to an extent. Vacationers come back with memories, which often included a visit to In-N-Out, which was an exciting and new burger to these people. Their excitement and bragging makes great word of mouth, thus a legend is born.
Choices are great, right? The more options you provide to your customers makes people feel special. But there is also much to be said about the In-N-Out menu, exclusivity shows confidence. Do you want a Double-Double, Cheeseburger, or Hamburger, and would you like fries with that. The lack of dozens of menu items tells people the food is so good, this is all you need.
Finally, the "not so secret menu" makes people feel like they are in a club. As previously mentioned, customers like choice. What they like even more than that is a feel of belonging. All these factors add up, and what happens is the making of a legendary brand. Even from something as simple as a common fast-food burger.