Good books, TV shows and games need a villain. The basic plot of the story is based upon the evil villain being stopped by the good hero(s). Most TV shows are based upon the villain being defeated in a single episode. At the end of the show, evil has been vanquished until next week's episode. The alternative to the episodic TV series is a story arc that plays out over the season, or in the rare case, many seasons.
Game masters also have the option of setting up their games as single episodes or with a plot arc. Game episodes don't normally resolve themselves in a single game session, but an episode could be considered a single dungeon, a single quest or similar.
A typical episode campaign will have the party clear out one dungeon and then move on to clear out another. There is not a lot of connection between how the party moved from one dungeon to another. Each dungeon brings on a new set of villains to be defeated. This is often refered to low fantasy style campaign.
A common plot arc campaign will have the party defeat a great villain that threatens all of civilization. At first the party will defeat the lessor schemes and henchmen of the villain, but eventually they will move up and directly attack the great evil one. A prime example of this would be Lord of the Rings where everything will be alright in the world if the One Ring can be destroyed, thus destroying Sauron's power. This is often described as high fantasy.
A disadvantage I see to high fantasy is the question of where to go next. What worthy adventures exist after the Great Evil is destroyed? Also, why aren't more people helping to destroy this great evil? I feel that it is tougher to keep a party moving on it's own with high fantasy. The party has to complete some task at a specific time to prevent evil from winning. Also, potentially the party could accidently derail the plot by by-passing certain elements. What would have happened to the Lord of the Rings if Bilbo had immediately destroyed the ring instead of holding on to it for 20 years? Another problem I see with high fantasy is that killing the party ends the world. It would be tough for a new 1st level party to gather up the clues and experiance needed to defeat this great evil.
I prefer low fantasy. I would rather have lots of wide spread minor villains that have to be tied together. While there might be a central villian, anybody of similar power or sufficient numbers could defeat them. I find that low fantasy adventures make more sense to me for an on going world than other genres. I can easily envision lots of small groups of people going off to fight the monsters and take the treasure. I can string together a series of adventures based upon a vague plot line of menacing tribes of monsters, lost cities and such rather than destroying the one artifact that threatens the good life.
Here is an over arcing plot line that I like for low fantasy. The party goes off to earn a living by exploring abandoned dungeons and killing monsters that threaten human-centric settlements. If the party is killed, you can start another 1st level group in a different dungeon and protecting a different set of settlements. As the party gains levels they start to hear about an Orc leader of the local tribe. Killing the local tribal leader will halt the attacks in that area for a few years. While they are defeating the Orc tribe in their area, other adventurers are doing, or failing to do the same thing elsewhere. Eventually leaders arise among the Orcs or other monsters that can forge an alliance between not only tribes of the same race, but between various races. This might not happen right away, and there is no time line on this event. The party, and other other adventurers will hear of this leader. Various groups will try and kill the leader, while others will try diplomacy to breed internal rebellion among the tribes. Defeating the leader won't stop the raids, just make them less effective. Even killing this new leader will only allow another leader to start his climb to the top. Accidently killing this future leader won't derail the plot line either. Another leader would arise.
The first goals of the party is to kill Orcs. That's simple. As the party gets more powerful and savvy, they can try and kill the more effective leaders by taking the fight to the enemy or offering a bounty. They, or other groups can try to use political alliances and bribes to get one tribe of Orcs to fight another tribe or kill the arising great leader. It is possible that groups of humans will be played off against each other just like the humans are trying to play one group of monster off against another group. The monsters could even be used as proxies by various human governments to fight wars they can't fight openly. The only true solutions to keeping the Orcs from raiding human settlements is to change Orc culture from hunter gatherers to settled farmers or kill enough Orcs.
Historical examples of this might be the inter-clan fighting in Ireland from the time of the Norman settlements until the 1600s. Antother example would be the French & Indian allies vs the British and their Indian allies.
This kind of plot is also hard to get off track from. The Orcs won't go away, but neither will the party taking a break have a long term effect on the Orc's actions. The party can take a break from basic hack and slash for other types of adventures without losing their place.
From what little anime I've seen, anime series are almost all based upon an over arcing plot line. A small group of characters must complete some great deed or protect the world from some great evil. In each episode the characters grow more powerful, defeat yet another minion of the great evil one and never get help from the common people. I wonder if anime could work without the quest or great evil. Anime seems to all high fantasy or space opera and not low fantasy. I suspect it is a convention of the genre, not a requirement of the medium. It does seem that anime could do a Star Trek type weekly episode or Hercules: Legendary Journeys weekly adventure.