Fountain of Knowledge

Sequels, like Children of Dune? Yeah, I dig those.

Sequels like American Pie 2, 3 (I've forgotten how many pies there are)? Not so much. Like Raven said, they're just cashing in on a profitable idea.

And that's just it, I guess. Sequels (or prequels) with the intention of delving deeper into the world(s) the author has created? They tend add layers of empathy and understanding. Take Harry Potter series, for example (the books -- the movies, not so much). I'm not sure JK Rowling wrote Sorcerer's Stone with the intention of creating a series, but as the novels progressed, the stories grew as well (and not just in volume). We get to know more and more about the characters we've come to love and root for (even Voldemort). The storytelling grew as well, maturing with each book.

Problem with a trilogy is that I have to end up buying and finishing all three books even though they're not good. I need to finish stories I start reading. Problem with epic sagas such as the Wheel of Time series is that sometimes, authors don't know when to stop, and if (and when) they die without finishing, sometimes the series get suspended just like that, and sometimes, as in this case, someone else picks up the torch, but it just doesn't feel right.

But I'm not sure if trilogies and series are considered as sequels. It's more like the author loves the world he's built so much that he wants to revisit the world, tell more stories about some of the characters, or the setting. I think Jurassic Park falls into this category. And I love JP.