Comment by Jasper_

PBR is not really a new rendering stack or a new framework, but a way of thinking about the asset creation process and shader development that allows for more of a separation between material models and lighting. There were a few things associated with the PBR trend that do not have anything, strictly, to do with PBR, but came along for the ride and helped things:

* The recognized importance of specularity in materials, outside of the ones that are obviously shiny. Something something, everything has fresnel. Look at your wood desk, or your matte walls, and you'll notice you'll see some glossy reflections on them.

* We switched away from art workflows that generated normal maps out of Photoshop filters, and towards programs and workflows that kept a much better idea of 3D-ness throughout the stack. Substance Painter, Substance Designer, and baking normal maps from ZBrush are a huge and understated part of the PBR transformation.

* We changed around our terminology, and put more floorboards underneath the rugs. e.g. in the before times, if you had a material that was "unlit", it probably just displayed as the diffuse color texture, and nothing else. That doesn't make sense, that's the reflectance properties of the material. So we changed things around to be lights-first, and a lot of things fell into place from that. Together with that, basic kinds of global illumination started becoming more and more mainstream through probes.

* We redesigned lighting and shading models to use more physics in them. Out went the observational Phong and Blinn-Phong, and in came the more empirical Trowbridge-Reitz and GGX. We fixed lights to have accurate inverse-square-law falloffs. We replaced (or are in the process of replacing) point lights and spotlights with area lights. And GI became a reliable tool.

Before the PBR revolution, things were far more scattershot, though there were definitely attempts at trying to integrate physics. Half-Life 2, Unreal 3, and Halo Reach might not have strictly been "PBR" by modern standards, but definitely had a lot of good ideas floating around it with regards to lighting.