11-Start off by making sure what the game is all about. Make sure you know why you make the game and what kind of experience you wnat the player to have
12-Establish as soon as possible your core mechanics and try to prototype it alot to make sure it's simple, fun and allows depth. Prototyping the core mechanics will usualy make it evolve and at the same time, it will uncover problems you haven't thought of.
13-Make a design doc. It can be very simple, featuring only the rough idea you want the game to be, but having one makes a difference. it allows you to keep focus during the process of creation.
14-Plan before you code. Think about what you're gonna do and why your doing it before you start coding it. You'd be surprised of how many ideas turn out to be bad ideas while still on paper. I'm not saying to write everything down but the core logic should be on paper so you can see the general flow of your game before you start creating it.
15-Once you got your core game mechanic down, find secondary mechanics, such as variations of your main mechanic. Then, make a list of all sorts of challenges that use these mechanics. From jumping on an enemy to using a double jump. Anything you can think of.
16-Establish an economy: think of everything the player will be able to exchange with the game while playing. Coins, points, stars, whatever... Just list them
17-Give a meaning to wahtever you give to the player to play with. Every weapon, every economical element must have it's sole purpose. If they don't have a good reason to be there then they shouldn't be there at all. Being cool is not a good reason
18-Think about goals and what the player has to do. Make sure your game has plenty of long-term and mid-term goals
19-When building the oustide of the game (menus, title screens, etc...) set a standard that you will use throughout the game. Be consistant! If Enter means yes, it means yes all the time. Follow the standard that the player is usualy used to or establish it with him as soon as possible (ex: ESC means escape or exit not Ok or Accept)
20-Always make sure the player can access the game screen within less than three to five screens.
21-Always tell the player what's going on when he's not playing. if the game is loading, say so. If it's saving, tell him.
22-Whenever you do something that could alter the player's game in a bad way and that is not part of the gameplay, ask a confirmation (Ex: Leaving the game, overwriting, Deleting, etc...).
23-Whatever you do, make sure you don't remind the player that he's playing a game. Try to make everything part of the game. Try to find a way to make the interface look likes it a natural part of the game, not just tacked on (this follows the transition point Rinku gave.)
24-Don't use fonts you can find in windows. Find or make a font that fits the game well and try to stick by it as much as possible. You can vary the fonts used in a game but be careful: using too much different fonts won't look good. Try to associate one font to certain events so that the player can associate the font with certan events. (ex: messages from the game always use one font but npc messages always use another)
25-Whatever happens in the game, always make sure there are at least two feedbacks: graphical and sound (like Rinku mentionned) The player has to know, in as many ways as possible, what happenned. In some case, add texts.
26-Establish your game universe before you establish your story. This will help you to come up with better levels, storyline and of course, make the game feel natural. Explain everything about the elements the player will see. Why are there goblins in the wood? Why is there water in that cave? Why would someone live in a place that looks like a maze? The player will think about those, you must provide the answers.
27-If your playtesters are confused about something, try to figure out why. Your goal is to make something that everyone should be able to pick up. You have to adapt yourself, not make the player adapt himself to the game. If your core controls aren't working well, then ask yourself why do the players don't feel like it's natural and make it natural. Find the reason behind their comments and don't follow their comments without asking why first.
28-Games are made for your player's fun, not yours. If you're the only one understanding it or enjoying it, re-think your game. Too many times I've seen people creating a game for their own pleasure and it usualy ends up being crap. And then they cry wondering why it didn't work out.
29-A cool feature in one game doesn't mean it will be a cool in yours. It's okay to inspire yourself but just taking features from other games because they were cool might simply just spoil yours more than anything else.
30-Music is a powerful tool. Use it wisely and remember that silence can be a powerful melody when used correctly.
That's all I can think of right now... I have more but I gotta go grab a bite.