One of the best aspects of Marvel's Agents of S.H.I.E.L.D. is that the series is structured much like the comic books it's based on. Each season follows an "event" storyline that builds up into the ones that follow. The result is a series that brings the fans back season after season to see what's going to happen next, which popular characters from the comics will make an appearance, and how closely the series relates to the books.
Marvel's Agents of S.H.I.E.L.D.'s story arcs are called pods, and most seasons have one large one while others have two to three. Pods have been described as "7-10 episodes [that feature] little 'enclosed plotlines' that formed a larger whole." This structure will continue as the show enters its sixth and seventh seasons. Each pod has its fans, but some are considered significantly better than others.