Stuff. We want it, we need, we crave it. More to the point, we buy it, but the way we buy our stuff is changing rapidly. Even the nature of the stuff we buy is changing. For example, we once bought records at a record store. Later, we could buy them at the record store or from a big box store. Then, although by this time it was CDs, we could order them online and have them shipped to our door. Next, we could make the purchase online and download the data. Moving on, we were soon able to buy the rights to the music and stream it from a central server. Now, many people don't even bother to do that, since there are subscription services that let them listen to whatever they want, whenever they want.
I'm none too keen on some of these trends, as it happens. A lot of companies are starting to remind me of the bully in my freshman college dorm, who used to “borrow” my CDs indefinitely, saying, “anytime you want to listen to it, just let me know.” He was a jerk, but I learned something from him: if you don't get to keep what's yours with you, it isn't really yours.
The point, however, is that there are a great many threats to current brick and mortar retail stores. Before I point to the chains which will survive them, let's take a look at what the major threats are.