The posts were on the topic of writing a series, and what both of them did was gloss blithely over the inherent difference between a limited and an open-ended series, equating Harry Potter with various mystery/thriller series without ever taking note of the fact that they are comparing two different creatures. And the thing that makes a limited series different is something that is very near and dear to me: THE END.
A limited series has AN ending; it structures itself around that fact. Each part of the story evolves some distance toward that point, usually with an ascending sequence of climaxes that have the same overall rhythm as any single volume. All of the plots go to the same place. Because the previous volumes are essential--it's all one--you can limit the amount of refresher information you give readers, or section it off in an introduction. It is expected that characters, even the world will suffer major changes; that is kind of the point.
An open series has MANY endings; each book has its own. Each book's climax tends to be at the same pitch, and each new volume hits the emotional reset button. Without an end to aim for, subplots tend to proliferate; killing them off becomes a major commitment, deep-sixing a resource that could be used in future books. Either every book has an all-new major antagonist, or the major antagonist is hidden in the shadows and only his or her catspaws are defeated. The main character tends to be the draw for the entire series; therefore, it's difficult to make substantial changes to the character. Since you never know when you might pick up readers, there tends to be repeated descriptive information for new fans who don't know what the main character looks like, or what kind of car is the villain's signature.
In case you can't tell, I have very little use for open-ended series; it's one reason I hate most television (major exception: Babylon 5. Guess why.) A story that has no destination is doomed to lose its way and drown in a bog, and by the time it does that, no one will miss it.