I'm glad you asked. Here is a list of things The Witcher 3 has that Red Dead 2 doesn't:
- character abilities (even Call of Juarez: Gunslinger had them so it's not just an RPG thing)
- more complex combat system (sure dodge rolling was stupid but overall TW3 had different attacks, magic signs, potions, and bomb; Red Dead 2 has extremely basic gunplay inferior to above mentioned Call of Juarez)
- meaningful armor and weapon crafting; so far you can only do cosmetic upgrades such as wood stock and engravings that don't affect the weapons in any way even though the game calls for an upgrade system out of The Good, the Bad, and the Ugly.
- the above mentioned crafting system is tied to hunting so hunting is both important and meaningful; here the only thing I can do with the skins and body parts so far is to decorate the camp and get better satchels to carry more things, but even after 6 hours of playing I can't get any satchels done because I didn't do some random activities yet like upgrading the medical wagon twice.
- an actual dialogue system, so I get to pick the lines I want to say
- choices within quests