Game designer Ian Fisch at Gamasutra writes:
"If you look at which games succeed and fail critically, you will notice the outcome is generally not due to a great mechanic or a new idea, but from competent execution from the moment of boot-up to the end credits.
Countless games such as Spider-Man 2, Stranglehold, and Assassin's Creed present the player with innovative concepts but fall apart when it comes to filling up the 10-15 hours of gameplay.
On the other hand, you have games such as Call of Duty 4, which brings almost nothing new to the table, but excels greatly when it comes to pacing and level structure -- keeping the player hooked throughout the entire experience. Even games considered to be innovative -- Portal or World of Goo -- would not have had their success were it not for superb execution. "