* Your PC is only cool if he is cool during play, not because he has an amazing backstory. Likewise, your NPCs are only cool if they do cool stuff during play. Your adventure is only good if it provides good times to people playing it.
* The story does not have to be cool or make sense to anyone other than the people playing the game. Like in real life, you go on adventures to have experiences, not to tell stories after the fact.
* Different people like different things, including mechanics. Some people will never spend inspiration or that last potion of healing, no matter what you do. Some people want to pick fights, some people want interaction, some people want to play ninjas. If it suits them, that is okay.
* Find the best system for you and your players. Eventually, this will be the system you have created yourself.
* That puzzle (or conspiracy) you built for your players is not as obvious as you think. The players are not in your head.
* Everybody will forget most of the details after a couple of days. If you want long arches and complicated plots with various adventures, that is fine, but don't expect you player to remember every NPC they meet, unless they are recurring. Also, if something happens to the PCs - specially if they are wronged - they are more likely to remember.
* Every important person, thing or location should have ONE obvious distinction. Not grey hair, but a mohawk. Not a scar, but a distinct lack of nose. Not grey houses, but impossibly tall spires. Think "caricatures".
* Do not plan the story in advance and do not keep safeguards against derailing. No fudging dice, no saving the players from bad luck or bad choices. You're robbing them of some amazing experiences. Failing is part of the game.
* Few fights should be to the death. People are more likely to surrender than to fight to the bitter end, and few animals will take a beating if they can escape.
* Common sense trumps the rules. But if the rules defy common sense all the time, you should be looking for a different set of rules. This is about rules as physics, not story - people defy common sense all the time!
* Let the dice push you out of your comfort zone. Your PCs all failed their saving throws - now what? Your important NPC was killed before he could start his plan - what happens now? If you rely on common sense to decide probabilities without using the dice, everything will become predictable.
* Everyone said that already, but expect the unexpected from your players. Do not assume they will be nice to a baby in the crib when they are invading a castle.
There is a story to the last one, of course, but I'll leave it to another post.