Make or Break of Anime: Overarching Storylines
They seem to have taken a bit of a backseat in recent times. Does a show need one to be entertaining or, going further, fulfilling? Can a show get by simply on its wit or action, or does it need a strand of thread running through to connect the madness? I'm going to look at some specific examples through the course, and naturally I'm mainly referring to your 12/24 episode series as ongoing ones flit between main story arcs in the same way as your shorter series progresses from episode to episode.
In short, an overarching plot gives a show purpose and direction. The characters aren't falling into these crazy situations for no reason, and you know there's going to be some closure at the end of it. There's also that certain kind of drive that only exists where there is movement towards a conclusion on a grander scale than those of individual episodes. But all of this only comes together when work is put into the main storyline; it's not just there for the sake of it.
An overarching plot line can often be the most interesting part of a show, and it can give you a reason to carry on watching even if a particular episode or two isn't quite to your tastes. This is because it's easier to create an engrossing and gripping story when you have more time to develop it properly and when ties to characters and setting have already been established (in some cases). So if you boast a story that plays out over the course of the show that's not interesting, even if the episodes are entertaining in the short-term the series isn't going to attract much of a dedicated audience. Similarly if a main undercurrent is implied but then never referred to then it has failed in its purpose - i.e. to supply (at least a sense of) progression - and thus it is pointless to have it there.
The way I see it, overarching storylines are an all-or-nothing affair. It really grinds my gears when a show introduces this important story arc that supposedly overshadows everything else that's going on, and then just occasionally tips its hat to it as it goes about a whole load of unrelated junk. This has been happening quite a lot recently, which is part of the reason for this post. Let's take a look at Hanasaku Iroha. Can you tell me if there's a main plot that runs throughout the show? No? I can't tell either, and that's really not how you want it to be. It's been argued that the question of who is going to inherit Kissuiso is supposed to be some kind of overarching plot, but that's only really been looked at a couple of times and only in the more recent episodes. Not to mention that doesn't seem to have anything to do with our protagonist Ohana, at least not for another generation. I think the show would work perfectly fine as a series of occurrences at the inn after Ohana has moved there, as it pretty much has been, without trying to weave some semblance of a main plot into it. Otherwise most of the episodes seem a tad pointless and/or detract from the story we're meant to be following keenly. The series isn't crying out for progression, per se, it's more coming from a slice-of-life angle now that Ohana has settled in.
Next up is Ao no Exorcist. Here is a lesson for you all, so listen well: Having two characters sit in the background of random scenes and chat exposition at one another whilst something completely unrelated is going on does not count as an overarching storyline. In fact it's just a tease: the show saying "Hey, we could have had some kind of progression here, but look at all the fun those characters are having!" Give me some actual content to back-up whatever it is Mephisto is on about and I might actually credit you with substance.
So you really have to make up your mind. Decide to dedicate your show to a long-term goal, following from a series of build-up and anticipation (with the odd filler, granted), resulting in viewer gratification. Otherwise just take every episode as it comes - if you don't have an engaging progression to link events to then don't try and link them. "Characters go to school, hijinks ensue" is more than enough of a premise to carry a series through without introducing burdensome exposition, as you see particularly in comedy shows - Lucky Star being a prime example, or from the current season Sket Dance works in very much the same way.
An anime can be a success regardless of whether or not it has an overarching plot, just so long as it approaches (the lack of) it correctly. If one exists then there has to be some kind of development followed by some kind of conclusion - you don't want to leave the viewer without closure, and you definitely don't want them to forget the supposed point behind the show. As such, the underlying story can really make or break a series.