Here's something you'd have heard far too many times by now regarding film sequels - "They're never as good as the original. The producers/developers are just cashing in on previously earned credibility"
Game sequels fortunately don't follow the rule of film sequels (though there are some notorious exceptions to this rule :P ). For the most part, they're better than the original game because the developer has learned what works, what doesn't and what needs to be improved to turn a successful game into a franchise. Game sequels are often bigger, improved versions of the original game, rather than continuations or progressions.
Although there's nothing wrong with including "more of the same", sometimes developers just don't tae sequels to their full potential, much to the disappointment of fans. Film sequels seem to be the exact opposite: more often than not they fail to meet expectations. At least we can look forward to playing some of our favorite characters, or discover new twists and turns, or better gameplay.
Admittedly, sequels in any narrative medium are hard to pull off, as their creators wrestle with the task pleasing fans and at the same time trying something new. How much of the original work should carry over into the sequel? How do you satisfy your fans, yet, at the same time, attract new audience? Sequels get a bad rap from most quarters. People still moan and groan about the industry's lack of motivation and innovation and how they stick to the same tried and tested formula. And you have to admit the 15th RTS of the year can be a bit tiring even for the most hardcore gamers.
Impressions games is famous for its city building clones. If you've played Caesar, Pharaoh, Cleopatra, Zeus, Poseidon, all of them use the same graphics engine and are minor modifications of the original Caesar game.
There is no reason to complain about repetition form non-linear sequels. Sometimes the title is enough for you've come to trust it over the years. Final Fantasy starts with new character and a new story, and the only thing that stays same is the gameplay. The Quake series on the other hand didn't have anything much in common apart from the strong multiplayer mode.
Of course, only successful games have sequels. But, there are many classic games that have received critical appeal but somehow fail to impress on the store shelves. These are games that influence us all, wind up on the cover of magazines and game sites, but elude the interest of the general public. Sin, for example was a game that had it all; hype, promise and even the media backing it, but was forgotten and its competitor Half-life remains even today among the top-grossers.
People who complain about lack of innovation in games fail to understand that innovation doesn't always pay. To start a new game from scratch, develop it for several years; only to risk rejection, is pretty dangerous. Which is why most of the game developers make one huge effort, and then consolidate their position with sequels.
For all the whining and complaining, the vast majority of gamers don't actually buy that many original titles. They look for familiarity and treasure it. New gameplay mechanics can frustrate the hardcore factions. When factors lie the speed of a rocket launcher can be the cause of endless debate, too many changes can lead to a change in loyalties. Which is why sport franchises lie FIFA, NBA Live, and Madden cash in on the franchise with new updates every year. They offer nothing but cosmetic changes, updated rosters and minor tweaking in gameplay.
At the end of the day, while one may look for a new gaming experience, it's the promise of familiarity which draws the larger crowd. Though originality is sacrificed, the first innovation is pretty much enough to take the franchise forward. And you have to admit that over the years the gaming industry has seen some fantastic sequels. The Grand Theft Auto series is living proof of this fact. Just goes to show, sequels aren't all that bad ;)