Dialogue with characters, quests, and the ability to create and role-play any kind of character you wish are identical. The ability to broadcast messages in order to communicate with players. There is no acceleration of time, so no world map will be available to speed travel.
Maps are smaller and built especially with multi-player use in mind.
A rather unique aspect of multi-player gaming in Arcanum is the ability to set up different types of sessions by changing a few parameters. The person hosting the Server will be able to set the game up as either a Free For All, a Cooperative, or Team. The type of game specified effects how combat between players is handled. In a Free For All game, any player can damage any other person playing online. During a Cooperative game, players will be unable to target or attack other characters.