Again, there's a big difference between shows based upon existing media (like a book/comic book series) and shows that wing it.
Shows based upon existing media have all of the hard work done for them, because of course an author will have set up foreshadowing, made sure the middle portion of a book is interesting, etc. They can even in some cases improve on what the books do by coming up with, treating the book as a "rough draft."
In contrast, when shows wing it, they have two choices:
- Write in a linear fashion from beginning to the end. This could result in the season seeming good up until the final act, when you realize you've backed yourself into a plot corner.
- Write with a predetermined ending in mind. When you do this, the payoff should be good, but you have to spend a lot of time wheel spinning in the middle. It can also hurt characterization, because actions in the middle act(s) have to move towards the predetermined conclusion even if it results in out-of-character moments (plot railroading, more or less).