#6. Thou shalt not pad the length of your games.
Mass Effect, The Godfather games, Legend of Zelda: Twilight Princess, Elder Scrolls: Oblivion IV, ah, fuck it. Any open-world game this generation.
See, here's the thing. We don't mind short games. Portal was a short game, everybody loved it. It was four hours of joy. Short is fine, as long as you adjust the price accordingly.
What you have started doing instead, game industry, is taking your short game and inventing some arbitrary way to pad the length. Such as:
Putting huge stretches of land between objectives.
Wow, what an awesome sprawling landscape your game inhabits. So sprawling that we have to ride a fucking horse for 20 minutes to get to the next mission. You also make it so that it's often not clear what the next objective is, and thus we must wander around aimlessly until we stumble across it. You then add up all of this cumulative horse riding and aimless wandering and boast that your game has "50 hours of game play."
"We must reach the citadel! It's 800 miles that way."
It's padding, plain and simple. And so is ...
Adding pointless, mandatory fetch quests.
The Metroid Prime series is guilty as hell of this, letting you get near the end before you have to track back across all the old levels and retrieve a bunch of shit. Twilight Princess turned us into a dog and made us go retrieve magical pearls for what felt like days at a time.
Games like Oblivion and Mass Effect give the illusion of almost infinite length, but their endless "go into another identical dungeon and retrieve X" side quests are just slightly remixed copies of previous levels.
Those games get a little bit of a pass because their repetitive side-quests are optional. Which brings us right to ...