The first and most important rule is to make the world come alive and lay the foundation of the game's design philosophy. The world of Tyria has been thoroughly established, from geopolitical histories to cultural development to ecosystems. This kind of planned-out depth makes certain that game-world elements added in throughout the design process fit in the world being created and that the game has a consistent and coherent feel, which adds to its liveliness. The second rule, that cooperation is key, is behind mechanics like individually tracked resource nodes, a universal revival ability, and loot and XP rewards for everyone who helps kill a foe or complete an event.
Allowing players to play the game rather than the UI is the third rule. This rule is responsible for the lovely and minimalistic UI, which is meant to convey as much information as necessary while staying more or less out of the way. Important combat information isn't necessarily going to be relayed through the UI so much as through the game world. Taking risks, ArenaNet's fourth guiding rule, allows devs to iterate and try new designs without being terrified of failure. Not all ideas work out, but learning why they didn't work out is key to growth.
The last two rules, to "do it well or don't do it at all" and "respect the player," go hand-in-hand. The first is about making sure that everything in the game world is as solid and well-done as possible and that there is a consistent level of quality in everything being put out, even if that means that smaller "nice to have" features get cut or delayed until after launch. The last is about the devs being proud of everything they create and certain that what they're putting out is worth the time that players will pour into it.
Now you know, and knowing is half the battle.