PBS Game/Show made a pretty interesting video on the subject that's worth a watch.
We should support our favorite creators for how they create, not always for what they create. In film, you might be a fan of Kathryn Bigelow or Quentin Tarantino, but that doesn't mean that you expect them to make 10 sequels to Point Break or Pulp Fiction. ... It's important that we give game designers the exact same latitude.
I've never really agreed with this underlying idea that sequels are bad though, to me, execution of the idea is more important than it's originality. Much used themes and ideas can still make great games, games with original and fresh ideas can still be crap. Still, not a bad video, and relevant given the increased bleeding over of System Wars culture into this forum. (something that really is going to have to stop)