I love In-N-Out... They have my favorite burger of all the burgers I've had... Originally being from southern California, I may be biased about it, but Double-Doubles are so amazing... And I eat a lot of burgers in a lot of places.
The sweetness of the sauce compliments the saltiness of the patties, contrasts the mildness of the cheese, compliments the crispness of the lettuce/tomato, contrasts the chewiness of the buns...
The fries are great too. Not overly crisp, not soggy, lightly salted, fried in vegetable oil, and cut to order.
They just stick to what they know and what the do well. Not over complicating the menus and keeping it simple. (even the "secret menu")