Call of Duty 4 is, in many ways, exactly the kind of game that describes the best possible sequel; those that are instantly familiar but obviously unique in a simultaneous paradox of fun. It doesn't simply rely on some improved graphics and a new map or two, but elevates the experience built on the foundation of prior titles. Developer Infinity Ward had a firm understanding of what defines a Call of Duty game, a strong foundation on which to build, and was able to turn the power of the franchise into a step forward by changing its setting and tweak its mechanics, yet still create a game that belongs in the franchise.
Modern media is built on delivering long-term entertainment experiences. We have television shows that build a single narrative over numerous seasons. We reward films that can create cohesive trilogies, and read books that take a million words or more to tell a single story over a half-dozen volumes. Sequels in videogames are a natural extension of that kind of consumer-driven entertainment, and in an industry where costs run so high, being able to provide developers with existing tools allowing them to focus on fine-tuning the experience is often a benefit to the gamer.
Even Mario, whose tired and tattered franchise has been dragged through every conceivable marketing angle, can still manage to charm with Super Mario Galaxy on the Wii or New Super Mario Bros. on the DS. Sequels have the power to take developer creativity and amplify it, even in the most established franchises, because the innovation in games is more important than the branding, and new life breathed deep into the bellows of a familiar franchise can create some of the most powerful games on the market.
I long for the next Diablo, the further adventures of Gordon Freeman, to see what Bethesda can do with the Fallout franchise, more deity destruction with a PS3 God of War, to bring down star destroyers as Vader's apprentice in The Force Unleashed, and, yes, even the charming Mario game to come that validates the haphazard trash of most Mario cash-ins. The price of great sequels is the same as the price of great new properties: the frequent and predictable suffering of mediocrity. For every Quake 3 there is a Quake 4; for every Civilization IV a Civilization III; and for every Star Wars Jedi Knight: Jedi Academy there is a Star Wars: Force Commander. As usual, to focus on the negative is to miss the point.
Sequels are not, by default, the absence of creativity or lazy development, but an engine for the continued success of a growing industry and a comfortable home for players. Innovation does not happen only within world-building, but rather is most often reached by building on foundations already laid. We enjoy the fruits of some extraordinarily creative minds, and whether they ply their trade in new worlds or old doesn't seem to be the measure of greatness or success.