The Nether is my shadowy/misty/ethereal plane. It was pretty much fated to work out that way, if I think about it.
The 4e plane of shadow is a place. It’s a gloomy place, with a lot of undead. It’s got dirt and trees, though. Oh, and it’s associated with death and dying, but it’s not like any rules really underscore that; for instance, ghosts don’t shift there during a haunting (unless I missed an awesome adventure or something).
The 3e/5e ethereal plane is a transitive plane. It’s “backstage” of the world: you can see the world hazily, but you can’t (generally) interact with it. Ghosts and phase spiders, maybe other monsters, are tied to it. But most undead aren’t; they kept their shadow plane-y associations, and so ghosts are somewhat unusual in that they explicitly go ethereal. And in fact, nothing goes shadowy; in Volo’s Guide, we got the planar-associated Shadow Mastiff, and its rules interact with the Ethereal plane (NOT the Shadow plane).
So, wham, they’re combined. The Nether Plane is like space, empty. Also like space, you can move physically away from the world (“up”), moving away from everything until surrounded by the void. And sometimes, if you are very lucky, find somewhere new, leaving the Deep Nether and finding a new place.
It has currents, so it is sometimes described as a river (or even multiple rivers). This is where the Styx is, and the other four rivers as well. Sometimes the current is so strong that the Nether seems almost solid, but it doesn’t have material; the current doesn’t seem to be manipulable in any useful ways by anything less than magical coercion.
Charon, the boatman of myth, definitely has equivalents here: pressgangs of devils, headhunter valkyries, scavenger demons. The currents play a huge role here, since the dead tend to be confused and unconscious, so get swept into these machinations quite easily.
They also lead to a nautical theme. If I want my spelljammers, my extraplanar ships, they’ll be Nethercraft, sailing the land of the dead from world to world. Gates and worldfalls are probably manipulated via enormous human sacrifices, altering the flow of the Nether and bringing places into (or out of) conjunction.