Things I learned listening to Half Life 2: Episode One commentary
1) If you wanna listen to how things get done, expect to lose whatever comfortable illusions you may have had about the world.
2) Whenever Valve introduces a new mechanic, they train you in a safe environment, then slightly different to ensure you've got the idea, and finally give you a major threat so you truly have to incorporate this new thing. I should keep an eye out for similar procedures in other games.
3) If you let players feel clever by cheating through a puzzle, expect them to think you designed it poorly by missing the cheaty solution.
3a) Good playtesting is essential to the brand and company. Wannabe designers like me will impact how people think about your game. While we will exploit glitches to further our performance, people dislike their existence in general.
4) Too much or too little action fails in a game. Honestly, how long do you play through a game that doesn't shake it up constantly? Mario games have shown this well going back to Super Mario 2/USA. Thread the needle between new and old territory.
5) If you want players to take a certain action quickly, entice them with subtle visual cues or goodies. If you want them to sit still, either force them to stop (not recommended), or at least provide a good reason for them to delay.
5a) Some people actually like opening a door that hordes of zombies are trying to break down. I seriously question this logic.
6) The amount of options one has for difficulty adjustment are staggering. Damage, fire rate, number of enemies, duration of effects/items, even map arrangement can change according to how hard the player wants it.
7) Big boss battles require the capability of a theoretically invincible player to attack endlessly, if mindlessly. If this requires the only infinite ammo be for the basic pistol, so be it.
8) Never underestimate the importance of lighting, from appropriate colors to contrasting shades.
9) It's surprising just how much innovation is possible for seemingly subtle elements.
10) If you want to focus on an environment, have everyone around you SHUT UP!
11) Nobody likes Navi. Companions in games are best left following unless asked to lead.
12) Most importantly, regardless of anything you do, always listen to your playtesters and the public. You are doing something for human enjoyment. Humans have to enjoy it. Never forget your prime directive.