I think overplottting and trying to have grandiose storylines is the bane of "traditional" storytelling and adventuring.
I disagree. Both are valid methods of storytelling.
D&D as I see it sets the stage for how to write for a group of characters that increase in power and prestige.
Now, go watch ANY long running TV series. Pretty much the same thing. At most they plot out a season (a couple levels worth of plot). The next season, they invent a new threat, or bring back some element from a past season as a twisted new threat. See the pattern.
Yes, this is the best idea if you plan on running a game forever or writing a TV show that never ends. That's why most long running shows use the formula. It means that as long as they get renewed for another season that they have something more to put into the show. It also means that if they get canceled after 1 season that the show can feel "ended".
However, my favorite show is Babylon 5. It was plotted pretty much from beginning to end before the show even started. Which showed in the character development, storyline and a number of other things. It seemed to flow more effectively than other shows because plot threads continued from episode to episode and from season to season way more than most shows on tv. It got to more effectively use foreshadowing and growth to tell the story. Because of this, IMHO, it was more engaging.
You want to write episodic if you expect different players every session who don't know what happened the session before hand(like most TV shows do). You want to write in closed "seasons" if you expect to have to keep writing them ad finitum. On the other hand, if you know your game is going from level 1 to 30 and you know that you want your PCs to end the game fulfilling their Epic Destinies and saving the world, then you have the ability to plan in advance. You don't have to put in an overarching storyline immediately, but you can put hints and foreshadowing in.
I'm not saying that you have to know what the 2nd encounter in session 48 is going to be before you start the game. But take this as an example:
Plot - Orcus wants to conquer the world
Heroic Tier: PCs deal with some orcs who are troubling the nearby area. They eventually deal with them and defeat them. There is a hint they are working for someone else bigger than them.
Paragon Tier: The PCs slowly begin to discover that there are armies of giants and other creatures rampaging the countryside under the same banner. The PCs will eventually track down the person in charge and find out he is a priest of Orcus.
Epic Tier: After defeating the priest of Orcus, they find out he has already performed a ritual which is tearing a hole in the dimensions and will allow Orcus' hoards to spill out into the world and wreak havok. The only way to stop the ritual is to go into the tear and fight Orcus on his home ground in order to close the portal from the other side. They eventually beat Orcus in a climactic battle.
And that's all I need planned, a couple of paragraphs describing the entire adventure ties it all together nicely and keeps the individual adventures on track. I can flesh out the details as I go, but I know that the plot is about Orcus and I can keep that in mind. Perhaps early in the heroic tier they find a town overrun by undead and a strange black portal that goes away shortly after they arrive. Later, they can discover that it was related to the overall plot because the priest of Orcus was just starting the ritual.