I firmly believe that the audience is part of the art. Considering demographics may sound like only encompasses marketability, but knowing an audience is an important part of interacting with and creating an experience for that audience. Games are an avenue not only for audience participation, but audience creation. This is most easily understood as it applies to tabletop games, or games without the strictness of implemented code. It's the duty of the Game Master to incorporate the player into the game, and I believe it should start before the first die is cast.
I've been GMing two games that are coming to a close soon. One has been going on nearly three years, the other about two. Neither are perfect; I've made so many mistakes I long ago lost track of them all. Which is a shame, because learning from a mistake is its most important element. But let's start with the very first mistake I made, before I even met many of my players. That mistake was in making the game I wanted to make. I love my ideas, I love them so much. But everyone has ideas. So instead of beginning the game by selecting one person to have all the ideas, tabletops should really starting with a planning session, where every player can get involved. This may seem elementary, but I rarely see it happen in a useful way. Here are some important things I think need to be covered.
Tone. Is the game going to be light-hearted with lots of jokes and laughter? Is it going to be set during an alternate Steampunk 1920's time? Is it going to be full of sex, drugs, and rock and roll? How are you going to deal with those things? I think that getting to an established tone, and making sure everyone's on a similar page will give you answers to many of the questions that are sure to come after.
Censoring. What isn't okay in your game? A player wants to brutally mutilate a body, to describe it in gory detail. Maybe it's fine with you, but what about other players? Is sexual violence something that's going to be a part of your game? How graphic is it? It may seem silly at first, but it's important to sit down with your players and know what boundaries people just aren't comfortable with crossing. If you think they'll have trouble speaking up for fear of embarrassment, start it off yourself. What don't you want to see in your game? Further, if some things, like sex, are going to be in the game, how do you handle it? Best way I've seen is the characters get to the point right before sex, and we get a "fade to black" to represent whatever kind of shenanigans each players wants to think about, without pushing it in anyone's face or making it too awkward.
Game System. I know this might seem weird to people who use one system for every game, but different games and players demand different game systems. I don't want to play Monsterhearts unless the game is about slightly supernatural young adult emotions and relationships. I don't think Dungeons and Dragons (any edition) is suitable for a soap-opera style drama set in early 2000's Beverly Hills. The games is at your command, and it can be a ton of fun to find a system that's right for your game.
Character Creation. Everyone needs to be present for this. Pitching character backstories, concepts, classes, and abilities should be done together. This is going to do two wonderful things: characters can have synergy; not just with fighting styles and powers, but with motivation and goals. How many times have you adventured with a group of people that you character would never spend more than a minute with if they could avoid it? The whole "met in a bar" or "all in jail" are both wonderful at dividing the party right from the start. As an example: I ran a Cyberpunk game where all seven players (ugh) started off as criminals in jail. I told them to come up with their crime, and got everything from an angry daddy to child molester/murderer. Yeah. That sure was a ragtag bunch of scamps... End of the story, we played one session. And lastly, no secrets between players. Sure, the GM has to have secrets, but players needn't. Anyone capable of roleplaying should be able to divide what they know from what their character knows. Having no secrets allows other players to push a character's story forward, and still have fun and be invested in the ride. If only one player cares about a big reveal, then the reveal wasn't all that big.
Storyline. This will come out of character creation in a big way, but the players all want to tell their character's story. If you can be there while everyone weaves their stories together (and you should encourage them to do so), then an overarching plot can easily come out of it. As a GM, you may have been told that the enjoyment of your players is the most important aspect, and that you should bend over backwards to please them. You want to make them happy. I don't agree. I think that the players and the GM should be engaged by what they play. I think they should be invested and excited, but they don't have to be laughing the whole game. Your story can have many interesting twists and turns, and the players will most often be the ones to give them to you.
So take that as you will. I know I'm not perfect at this, but from here on, whenever I start playing a new game, it will be with all of my players, from the start.