Always been a fan of Raph Koster since UO, so decided to dig this out of his website and share, as even tho it's an old writing it is still fully relevant now in regards online multiplayer games.

Enjoy.

http://www.raphkoster.com/gaming/laws.shtml

My 2 favorite lines are

Never trust the client.
Never put anything on the client. The client is in the hands of the enemy. Never ever ever forget this.

Pleasing your Players
Despite your best intentions, any change will be looked upon as a bad change to a large percentage of your players. Even those who forgot they asked for it to begin with.