You're probably already familiar with this one:
"You can't please everyone all the time."
To this I add the following corollaries which I've found to be largely true:
"You can't please any single person all the time."
"You can't please all people at any one time."
Corollaries, postulates, and Axioms:
"If you're only going to choose one trick, DPS is a pretty good one."
"Dead mobs do zero DPS."
"Living through an encounter is a superpower for a DPSer."
"There are very few encounters that aren't solved by liberal application of overwhelming firepower."
"I can't {heal | DPS |tank} any faster! I'm mashing the button as fast as I can!"
"
"There are no unskilled positions." (Wildstar)
"The actors may have changed but the roles remain the same." True in business, too!