This may be the most important rule for making an awesome game, and it’s one that I most often see broken: the players are the main plot. Do not overshadow them with your NPCs.
NPCs are awesome. We get to spend hours creating five, ten, fifty, hundreds of them for the players to interact with. They get names, little backstories and flavor to make them memorable (more on that later)… but at the end of the day, the players are the movers and the shakers in the game.
This doesn’t mean the PCs have to save the world. You can have an awesome game where the PCs aren’t powerful at all – but the PCs actions should be in the limelight for the campaign.
This walks a fine line – it is okay to have powerful NPCs in your game. It’s okay for them to interact with the PCs. Just don’t let them take the focus away from the players.
Example: The Kill-Stealer
The players have finally reached the villain of your campaign. They just rescued a thief from the dungeons and he has been tagging along ever since. The villain and the PCs start fighting – but it turns out that thief was actually Prince OP (fabulous he!), a 20th level fighter with magic gear that he hid who-knows-where! He charges the main villain for imprisoning him. As the PCs try to get hits in, Prince OP simply crushes the main villain of the game with a Doomhammer – along with the players’ hopes and dreams.
While it’s an extreme example, variants of this occur in games all the time. The PCs look like they need a little help, but that awesome NPC you make to support them turns out to steal the whole show. There are plenty of ways to give them a little support, show off that NPCs have power too and still let the players have a field day.
Better Example: The Noble Fool
The PCs are in the same situation as above – but as they reach the main villain, they realize that he has backup! They can hear the clang of armor, the yells of the evil henchmen storming up the stairs and know they can’t face both the main villain and the henchmen at once.
Fortunately, Prince OP is to the rescue! He valiantly offers to hold off those henchmen for as long as possible – “Take care of him! I’ll fight these curs!”
This gives the NPC an opportunity to shine (and might even endear the PCs to him after the fact), shows that there are other people in the world who are willing to help… and lets the PCs have their moment against the villain instead of having it snatched out of their grasp.