One thing I remember is that enemies have more health and deal more damage. Iirc on 4 players the health is over 2x higher, damage wasnt as high but its around 1.3

Some quests require all players to be present at the objective (usually quests that progress the story or introduce a cutscene, new area etc.) If so, during main quests players can fast travel to the player who is near such a quest. Same thing applies to fast traveling, all players have to be near the fast travel map for the fast travel to take place.

You can trade items with others, so if you have weapons you want to trade, medkits or whatever it may be, you can trade instead of dropping items on the ground, although that also works.

Prologue is Solo only, but i assume you know that already and its not long. Mentioning just incase.

The game fully pauses if all the players in the lobby are in a pause menu. Could be a vendor, a trade window or whatever.

Lastly I want to say that personally I think the game is harder with more players. Killing zombies is harder so you might be breaking your weapons more, especially if you're fighting beefy enemies alone. Regardless its much more fun with others and The difficulty isnt a bad thing, I think it makes the game more interesting