-A games base mechanics should be intuitive. If its a complex game, then you make damn sure the techniques to get through the first hour or so are intuitive and easy to grasp, so you can gradually tutorialize the player.
-If your game has a story, any story at all, hire actual writers! I cannot stress this enough how important this is to me in games I enjoy. Hire writers who know how to write dialogue, and get them in on the process, so that the narrative is f**king paced properly.
-Testing, testing, testing. Don't just use playtesters to find bugs. Observe them. Where are they having difficulty? Where are they having fun? Analyze these parts.
-Sometimes color is nice. In fact, most of the time color is nice. Brown is a color. Use it like any other.
-If a game's base mechanics support multiplayer, than it makes a nice afterthought. But unless a game is built from the ground up to be online (see: MAG, TF2), than your campaign should be your priority.