1 will run on modern Windows, Linux and maybe mobile
2 will support modern screen resolutions and wide screen
3 will include engine bugs, include improvements like supporting more RAM,
Many say that Windows offer backwords compatibility, but many old games will not run right, though it could be the drivers.
You're supposed to have a licensed copy of the game for the non-redistributable content, but the code of QuakeWorld is modern, runs better, etc. as opposed to trying to install Quake 1 itself from Steam and hoping that it runs.
I mean just being an open-source version implementation of the engine seems noteworthy of itself, non? From mods, to alternate system support, etc that's all implicitly possible now.