I think the defining factor that separates STALKER from other open world action games is that in most of them the open world is treated like a container with stuff in it. A transitional plane from which you access different missions and parts of the actual worthwhile experience. In STALKER it's the opposite, as the open world IS the experience and the Zone is treated like the main entity, plot and attraction of the whole game.

It's not perfect by any means, and the slavjank jokes write themselves. But in a scene full of open world games that don't know what to do with their own concept, this old decrepit buggy ass game gets it.