There's an ongoing debate on the iPhone Dev SDK Forum about what is more important - the idea, or the execution? And I'd like to weigh in on the matter.
When Conan of Cimmeria was asked "What is best in life?" he replied thusly: "To crush your enemies, to see them driven before you, and to hear the lamentations of deyah women."
Much has changed since the Hyborian age, but success is still a three-legged stool, and the three modern legs are idea, execution, and marketing. You need all three, or you can not succeed.
The world is not lacking for ideas - every day young dreamers come to the Indegamer Forums with their plan for the world's best FPS with 400 gun types and realistic damage and AI that "really learns," while others post nearby with their ideas for the ultimate MMORPG with hundreds of quests and items. Veteran posters point them to a book on C++ and the dreamers quickly disappear, never to be heard from again. Tom Sloper at Sloperama is also quick to disabuse them of the notion that their idea is valuable. There is no shortage of ideas in the world, good and bad.
I'm going to anger anyone who fanices themselves an "Idea Man," but here it is: of the three legs, I'd say that idea is the least important. You can take a tired or derivative idea and still find success if you execute well and market well. Half-life was not the first FPS game, nor was StarCraft the first RTS game. Both are examples of excellent execution - and the companies behind both did heavy marketing before and after release.
The majority of indie projects languish once they get to the "barely playable" stage, where the developer has scratched his personal itch and figured out a particularly fun piece of code. That's really only 10% progress, and the other 90% is still ahead - level design, artwork, sound, music, writing (prose) and other content creation, then settings screens, help screens, tutorials, and balancing/play testing, followed by more testing and bug fixes.
Of course no one can buy your game if you never complete it, so many Indie projects die on these fields; and if you skip any of these steps you may wind up with a product that's difficult to market in the final leg. The idea is nothing if it's not executed, and executed well.
Marketing merits a post of its own (and it will get one) but for now let's just be clear - no one will pick your product out of the rough and trumpet its virtues if you're not willing to trumpet them yourself. No one is going to show up with the "standard rich-and-famous contract," so you need to put forth much effort here too. Howard Aiken's famous adage is right: "If your ideas are any good, you'll have to ram them down people's throats," and the marketing phase is where this throat-ramming begins.
Marketing must begin before the game is completed, and must continue after the game is released. Many good games have had poor sales. You are competing with other games - some better than yours, but many worse - so you really need to fight for eyeballs. Cliff Harris of Positech games has many tips in this regard. Although I think adwords are a dead end for iPhone games, and it's difficult to track conversion in iTunes, he has lots of other good advice.
The Threefold Path
So there you have it - it's pointless to argue whether idea or execution is more important, because the are not separate options. Along with marketing, they form *steps* not *choices,* and asking which one is more crucial is like arguing about the most important step on a staircase, or the most important rung on a ladder.
I'm glad to have made it through the execution phase with my game "Dead Panic," and hope to navigate the marketing phase as well.