The original plan was to have one code base.
The Odyssey players would be able to land at that new ground bases but the Horizons players just would not see them.
As stupid as that sounds, that's pretty much the only option they had, it was that or only put Odyssey bases on the thin atmosphere planets ( which would have made more sense, but they are more rare ).
I still think there will be too options in the launcher, just like there was for base game and Horizons for a long long time. ( various glitches where 'worked around' by reloading with the base game client )
So Horizons players will get:
1) new planet tech ( ie no more wonderful RNG planets, instead just the new cookie-cutter handmade assets scaled and rotated and tiled randomly - a major downgrade in my eyes ).
2) new material based textures and lighting - so no more driving around at 100+FPS on planets, you'll take around a 60-80% cut in frames!
3) new UI - The new UI is finally back to the levels of Horizons in teams of what you can do, but I still greatly prefer the Horizons Gal and Sys maps.
4) and as lefty1117 said "All of the beautiful dust and colour variation seems to be lost"
That's about it. Just be grateful you don't have to try and do ground combat at your new reduced FPS.
FYI: I solved Odyssey not having playable frame rates on the ground - I bought a new PC ( not to play Odyssey, but because the old one died

). The old PC got 40-50 FPS in Cyberpunk on good settings, even in combat, but Odyssey on foot was 30-35FPS at a ground base, with no combat and about a third of the level detail as Cyberpunk! Old PC at a Guardian site in SRV Odyssey got 40FPS, Horizons 110FPS. - I've only loaded Odyssey once since getting the new PC, just to test frame rates on the ground, it took about a hour to fully get my keys and controller bound correctly ( and I didn't even test SRV controls! ).