1st: Most important thing in any mmo: balance damage and survivability of all the classes.
2nd: Design content keeping all classes and their roles in mind.
3rd: Pets and weapon procs should only act as extention to damage/survivability, they shouldnt be OP enough to replace a class/role.

Both 1st and 2nd point go hand in hand, if a class is overpowered then irrespective of how mechanic driven and role specific the content is the op classe(s) just nuke their way through (which is what is happening right now).