One of the big ones is something the board has touched on briefly in other threads over the past year or so: have a mission statement.
Have one summary sentence. Give it a paragraph or two explaining what this means.
Everything else -- on game, wiki, or wherever this information can be found -- should support this statement, be it code, policy, RPG system, grid design, etc.
This is not the same as theme. Most games have multiple themes at play at any given time. Making a list of these concepts -- again, a single summary statement and a brief explanation is most useful here. This is where you talk about the kinds of stories you want to tell.
Policy should make sense. If your game is PvP, policy should support this. If your game is strictly collaborative and consent-based, your policy should be structured to support this. Policy is not one size fits all. This is where you talk about the way you want the community to behave in relation to one another.
The RPG system should do what you want to see on the game, and not do the things you don't want to see on the game. This may require house rules if you are using an existing system; make them clear and explain why they're there. If you're making a system from scratch, don't set it up to do things you don't want happening on the game.
The grid is your playground. If you're cool with people holing up in small groups, provide for this in some form. If you want to make every scene publicly accessible to all at any time, build the grid in that way instead.
This is all a matter of making choices, explaining those choices and what they will mean for the players, and maintaining consistency regarding those choices.