If you want a sequel that sucks, head to the movie theater, but it you want a sequel that rocks, pick up a controller.
The law of diminishing return dictates that when a studio announces a sequel to a movie, it's probably going to suck. Videogame sequels are curious exception to this rule however, and people actually expect them to be better than the games that came before them. In Issue 304 of The Escapist, Brendan Main talks about what makes videogame sequels unique amongst entertainment media, and why the expectations of improvements aren't wholly unreasonable.
There are three myths that inevitably pop up whenever the relative worth of various media are discussed: The book is always better, the adaptation is always worse, and sequels are always rotten ... When it comes to videogames, the first two rules still rule supreme, but the third is nowhere to be found. Despite all the sequel-bashing that takes place with other media, a videogame sequel is a different case altogether. Rather than assume that a sequel is an innately inferior thing, often the opposite is true - we expect sequels to increase in quality. In fact, this assumption is so commonplace that we don't often stop to consider how radical it really is.
Usually in other works, a sequel is an extrapolation ... And in this scramble to add on additional stuff, the original can be grossly mischaracterized or misunderstood ... It isn't just that a sequel might take a beloved story in a weird direction, but that in its missteps, it drags the first one along behind it through the muck.
But a sequel in a videogame doesn't necessarily work that way. There is no need to slap the one onto the other, suggesting a direct and chronological type of sequentiality. What it might do instead is reinterpret and reiterate, and serve as a mechanical sequel, rather than a narrative one.
By exploiting this unique feature of videogames - where developers keep a game's plot roughly the same as its predecessor, but significantly changes and upgrades what the player does - the medium can buck the trends that plague sequels in other forms of media. You can read more in Main's article, "Play it Again."