Character Customisation

You can be male or female
You select a base face and adjust things like your nose, brow, lips, eyes, hair and facial hair
It looks like hair growth is not a thing online
Your character is not voiced, like GTAO

Animals

Animals are online
Animals can be skinned and stowed just like single player

Horses

You can insure your horse, presumably so it isn't lost forever if killed - like mors mutual.
Deadeye
There are different cards which give your deadeye different abilities
i.e. target painting, or extra damage - these cards can be upgraded

Additional currency / micro-transactions

You can find gold nuggets in the game
100 nuggets = 1 gold bar
Gold can be used to unlock stuff early
You can buy gold bars with real money

Game Modes

It looks like online has a separate co-op story mode, the missions can be played co-op, and can be matchmade - co-op modes are 4 players
Freemode is seperate modes for different locations - for example, free roam lemoyne, free roam new austin, etc.
This doesn't lock you to a certain part of the map, it looks like it just changes your spawn location
32 player lobbies
Your posse can be either temporary or permanent - you pay $200 in game for a permanent posse

Camps

You can choose the location of your camp
Camps can be large or small
Camps have a flag - lower the flag to enable camp raids
Camps have a catalogue that you can order from - like a general store in singleplayer

Weapons

Weapon prices are higher online than in singleplayer

Misc

Honor functions online the same as singleplayer
It doesn't look like there is a drop in graphics, like there was in GTA5 and GTAO
Items are locked behind levels, so you must gain levels to unlock access to more items, like in GTAO
You can lasso other players!
Players show up on the map and minimap - your blip gets darker the more you kill players
There is blackjack and other mini games, it is not confirmed if you can play these with other players
You can be killed by predators
You pay a daily upkeep fee for your horses and your camp, like your garage and apartments in GTAO
There is proximity chat, it is a large proximity
You can go into a private lobby
You can attach horns to your horse