The Empire Strikes Back
Raides of the Lost Ark
The Good, The Bad and the Ugly
For a Few Dollars More
Big Trouble in Little China
It's a Wonderful Life
Smokey and the Bandit
Not quite the list most people would come up with, right? Especially Smokey and the Bandit; I almost felt embarrassed that I enjoy that movie so much because it has an almost non-existent plot. That's when I realized that almost all these movies, some of them considered to be the greatest films ever, have thin plots. I also noticed that Jedi wasn't on the list, nor Fist Full of Dollars. I like both those movies quite a bit, but in the case of Dollars, even though it's essentially Yojimbo (and thus a more detailed story), it's just not as appealing. I think this goes back to the simplistic plots of the later films.
To fully explain myself, take the plot for Smokey and the Bandit: Two guys bootleg some Coors and evade a sheriff. I could make several movies using this plot, and I'm sure they'd be terrible if that's all I had to go by, yet for whatever reason, the actual movie is funny as hell. The lack of intricate plot makes it much easier to get involved and to be entertained. A major literary device employed is in media res; we don't see Bandit and Snowman in their early careers smuggling goods or evading cops, instead we start right off with the action. All the background information is important, but only alluded to when it drives the film forward. In fact, every single film I listed above has a thin plot and starts off using in media res, except for It's a Wonderful Life, which sort of does anyway as the whole first half of the film is a flashback. Star Wars literally beings with a battle in space, the princess already having stolen the plans, evading capture. The plot to that movie looks like it should be: Rebels battle an oppressive Empire for freedom. But that's just the backdrop...it's really something more like a princess gets captured trying to help some rebels and is rescued by a farm boy and a smuggler.
Even though the plots to these movies are simple, the implementation of those plots is what makes them great. The characters are what create the story; the story does not exist independently from them. Too many times I have seen films with characters that weren't an essential part of the story, and those films almost always suck. If you take Han Solo out of the trilogy and replace him with a similar but slightly different character, you've destroyed the film (cf. Han shot first). Similarly, Hannibal Lecter is a classic film villain, necessary for Silence of the Lambs to be thrilling. Replacing him with a less intelligent character, or one who wasn't as quite as diabolical would make that movie really long and boring. To reiterate, characters make the story, the plot simply exists to give them a reason to act.
How does this relate to rpgs? I've seen a lot of adventures or campaigns wherein the DM attempted to come up with a good plot in order to tell a great story. This is not only unnecessary, but counterproductive. The plot to any adventure should be succinct; if it cannot be written in one short sentence, you've done too much. Further, in media res really is the best way to start any game. I've seen a lot of games start off with waaaaaay too much crap like "Okay so you've never met these characters before, do some roleplaying and see if you want to join up". This is followed by several SESSIONS of "getting to know" the other PCs. Stupid. Do it the Seven Samurai way: would you like to join us on this adventure? Yes? Okay, let's go. That's it. The primary characters already have reason to adventure and they drag in the others along the way. There doesn't have to be any reason for the characters to get involved beyond their curiosity. Or greed or whatever. Simple reasons work best, because in the end, they're the heroes and actions define heroism, not initial intent. Story should be something that develops naturally from the interaction of the characters and the plot. I do realize that railroading players is sometimes necessary to encourage action, but use the literary term plot hook and suddenly it doesn't seem so nefarious. Real life is full of plot hooks that cause people to act when they'd rather be drinking beer and watching football just like literature if full of plot hooks that engage the main characters.
If you think I'm being too simplistic, here's another example: Hamlet tries to discover who murdered his father, the king, and avenge his death. If the play were an rpg, Hamlet would show up in town wanting to kick back and drink ale, but the DM would say, hey, your father was murdered. That'd be the end of it. A simple plot hook. The resulting story would be a product of Hamlet being utterly incapable of making any sort of decision (classic PC behavior) and using divination magic to discover the truth, completely ignoring the obvious.
Complex plots are the province of amateur writers as they cover up thin characters. Good writers don't need anything fancy because they know their characters can stand alone, independent of the plot or story. Just like Indiana Jones is a great character outside of the movies, so too should the PCs be in any rpg campaign. By doing less as a DM, the PCs have more room to grow and shine, which results in much better stories. Plot is, indeed, overrated.