The advantage of TV over movies is how much room you have for plots to breathe. The most epic movie is going to top out around three hours long, so even a six-episode season of a British drama is going to have considerably more screen time. When you get into 10 to 22 episode seasons of the U.S., it becomes a very different way of telling a story on screen.
Not everyone handles this framework well, though: Introduce too many story threads, and some will get lost in the shuffle. Neglect the needs of an actor, and soon you might see him or her leaving the show, or give a major storyline to an actor right before their contract expires, and you can be left in the wind. If you shoot too much relative to a network's commercial commitments, you may have to cut something important. Conversely, edit the episode in a way that gives a minor plot thread minor greater weight than you intended, and the audience ends up thinking you didn't care. Or maybe they really just didn't care and got sloppy.
Way too many TV shows have, for one reason or another dropped major plot threads in puzzling, conspicuous ways. Some shows were notorious for it, but who stands out? Warning: All entries contain SPOILERS for the show in question.