Soren Johnson is a game designer and programmer who co-designed Civilization 3 and was project head and lead designer of Civilization 4. He's now working on Spore at Maxis with Will Wright and his team of geniuses. You might say Mr. Johnson knows a thing or two about game design, which makes his list of the eight worst game design mistakes very interesting reading:
Hard-core game conventions:
One of the most common pitfalls for a game designer is to fear that the game is not hard enough. This fear often leads to hard-core game conventions, like restrictive save systems and unlockable content, that only put roadblocks in the way of the mainstream gamer who is just looking to have a good time. If you feel your game needs the tension of a restrictive save system, go ahead and implement it... but only as a feature of a higher difficulty level. Difficulty levels are the key to making a game accessible to both the casual and the hard-core gamer; we could never seem to add enough difficulty levels to Civ to keep our wide variety of fans happy. Trauma Center (DS) is a good example of a great game that was ruined by having no difficulty levels whatsoever. The surgery game is a brilliant use of the DS touch-screen, but the linear challenges get so hard by the fourth or fifth level that most people get hopelessly stuck after only a couple hours.
Johnson has been maintaining his blog Designer-Notes since 2005. I encourage you to peruse the archives on his site and read his notes on the design process behind Civ4 and his thoughts on the structural underpinnings of many other games.