When I'm playing online with friends, how does the game handle lag? How does the game communicate with other copies of itself? Is there a dedicated server? How do I handle a player who is playing from Europe, and is connecting to a player in the United States? Should I only allow players to play online together based on their region?
Note: If a game does not handle lag properly, a player can use it to its advantage and cheat (or break the game). A famous game for this: Maplestory.