It might seem trivial, but showing one photo at a time is a design decision that creates more value for each image, and enhances your viewing experience. Plus it doesn’t hurt to have the images trapped inside a beautiful iPhone screen. It almost doesn’t matter who you follow—their photos probably look better one at a time. From a UX perspective, we keep learning that interfaces with constraints are successful, and it seems like such a straight-forward principle (140 characters, ahem), but it’s kind of worthless on it’s own. Obviously you can’t introduce constraints without other elements, which is why this is the last point. There’s something enticing about knowing that most Instagram photos are created on the iPhone, since it introduces a NASCAR-like equality. That makes it fun to see what other people can create with the same technical constraints you have. Photography has always been all about the equipment, and not at all about the equipment. Knowing millions of people are creating with roughly the same camera and app as you makes it exciting creatively. So constraints, combined with quality and an audience are what makes Instagram so addictive. (Hat tip The Fox is Black)