I know I can look it up, but what is Horizon's about? I'm pretty close to an Oculus Rift purchase and ED is one of the games I already own, but quit, and might get back into.
The headline feature is planetary landings, you can land on airless worlds, deploy a surface vehicle and drive about. Scout for materials, fight against drones, scan terminals at bases.
2.1 added Engineers, certain specialist bases let you do custom upgrades (much faster thrusters, longer jumps, more powerful Powerplants etc.).
2.2 added ship launched fighters, a handful of craft can deploy a fighter to fight alongside (you can switch between fighter and mothership at will)
2.3 added the ability to invite other commanders as crew onto your ship (they just zap into the seat, there's no walking around).
2.4 this month adds the Thargoids (dun dun dunnnnnnn)
Lots of other stuff too, some of which may be back-added to the non-Horizons version:
Megaships (massive non-combat ships, some of which you can dock with)
Guardian Ruins (ruins that you can explore, with a 'puzzle' you can solve for ~100m)
Thargoid Structures
improved spaceport internal variety (industrial ports look very different from agricultural etc.)
Lots of customisation options (shipkits, an avatar builder, ship name and ship ID decals)
Passenger missions
New station types (brought across from the CQC module) and Asteroid bases.
New star types (neutron stars and white dwarfs have tails that can be scooped for improved range).
A few new ships too (Dolphin and Beluga, small and massive passenger ships), and a few fighters.