If you know what a MacGuffin is, you're probably thinking that they actually don't matter in the slightest and that I'm full of nonsense. If you don't know what a MacGuffin is, here's a definition from Merriam-Webster: an object, event, or character in a film or story that serves to set and keep the plot in motion despite usually lacking intrinsic importance. The most famous example is the Maltese Falcon, in the film of the same name. It sets the plot in motion, in that every character wants the statue for himself, but lacks intrinsic value in that "the Maltese Falcon" could be swapped out for the treasure of your choice: the Abyssinian Monkey, the Peruvian Chinchilla, the MacGuffian Beast.
Technically, a MacGuffin only lacks intrinsic value in the meta sense, as the characters themselves obviously want the object for some reason. The Harry Potter books, I think, are perfect examples of well-done MacGuffins (except the last book, but more on that later). The Philosopher's Stone in the first book is a powerful magic item that grants perpetual life, so it makes sense for it to be under lock and key, and for Voldemort to want it. In the meta-narrative, it's just an object that provides a mystery for Harry & Co. to investigate. Rowling does something similar with the horcruxes, which are vitally important to stopping Voldemort, ie, are a convenient way to provide a challenging search for our heroes in the final book.
And yet, many authors don't treat MacGuffins like they matter. Because I overanalyze every bit of media I consume, I've noticed two trends of late wherein MacGuffins are misused, and I'm here to tell you all why this is a problem.
The first trend is when the writers forget why the MacGuffin matters to the characters. This occurs when a plot revolving around a MacGuffin stops dead in its tracks, sometimes for chapters or episodes on end, for no discernible reason. Now, I can give some leeway to dawdling around in comedies, since their purpose is to provide laughs rather than tension. In Slayers Next, for example, the characters are looking for the Claire Bible, a MacGuffin of untold power, but spend 90% of the plot derping around getting into ridiculous situations. To be fair, most of these situations are, in fact, because they are looking for the Claire Bible but keep getting false leads. The only arc where they actually stay in one place for multiple episodes is when they learn that the father of one of the characters has been assassinated and have to investigate. Notice, something intrinsically important—solving the murder of an important character—briefly takes precedence over finding the non-intrinsically important plot device. This diversion works, first, because the diversion is deeply personal and, second, because finding the MacGuffin is not time sensitive, as far as the characters know.
On the other hand, you have Harry Potter and the Deathly Hallows, where the characters camp in the woods for-seemingly-ever. This isn't a comedy, and what's more, lives are at stake. Every minute that Voldemort isn't killed is a chance for him and the Death Eaters to murder muggles and muggleborns. Yet, we the audience are treated to five million chapters (perhaps I exaggerate) of the characters sitting around wondering what to do next. What's even more frustrating is that the non-camping-in-the-woods parts of that book are so interesting: infiltrating the Ministry of Magic, escaping Malfoy Manor, breaking into Gringotts, returning to Hogwarts. I will be the first to say that a story needs downtime as well as action, but the chapters in the woods felt not like downtime, but wasted time: we learned that Ron has feelings of inadequacy, as well as a strong loyalty to his family, which we already knew; we learn that Harry is willing to brave freezing water to get important stuff, which we already knew; we learn, retroactively, that Snape's patronus is a doe because he loved Lily, which... Wait, why would he associate her with the female version of the animal her husband turns into? As usual, I digress...
Anyway, these chapters make the plot stop dead, making us wonder why the characters aren't doing more to find the horcruxes. They know some of the horcruxes are associated with the Hogwarts founders, so why not seek out a historian? They know they need some serious power to destroy them, so why not seek out a powerful wizard? Why not do something? You could easily argue that they were trying, but I'm not blaming the characters, I'm blaming the writer. She gave us these MacGuffins, but rather than use them to drive the plot, she lets the plot slowly, agonizingly crawl forward due to happenstance. The characters happen to overhear that the sword in Lestrange's vault was a fake, and then they happen to have the real one delivered right to them. Harry happens to be correct in his unsubstatiated theory that Helga Hufflepuff's cup might actually be in the vault, and then they happen to fly to Hogwarts where it just so happens that a ghost knows where the diadem is. Although they do take action once they know where a Horcrux is, they do nothing to actually try and find out where they are. Voldemort showing up at he end with Nagini in tow, by this point, is a convenience rather than a threat, because they don't have to bother finding him!
A more recent example is in Tangled: The Series, season two. The characters have a MacGuffin: nigh-unbreakable black rocks that are spreading across the land and that have a magical connection to the flower that saved Rapunzel's mom during childbirth. The characters are questing to find out what, exactly, these black rocks are all about. Much like Slayers, the series is rather lighthearted and episodic, so it's okay for them to take random side adventures here and there, since they are traveling and thus encountering various people and creatures. What is not okay is for them to get shipwrecked and stranded on a island for three episodes! The writers chose to do this, chose to have the characters stay in one spot rather than journey toward their goal. Like the horcruxes in Harry Potter, the black rocks are somewhat time sensitive, as they've eaten up swaths of land that people need for farming and living and whatnot. There is also a certain side character who shall remain nameless (no spoilers; watch the series!) who very much needs them to figure out how to undo the damage the rocks have done, ASAP. Dropping the main characters on the island makes it seem as if the writers forgot that all those people affected by the rocks exist. Yes, technically the MacGuffin doesn't have intrinsic value—they could be purple rocks or black thorns or whatever—but whatever they are, they do have value to the characters in that there is a pressing mystery to be solved.
And that is why dawdling in a MacGuffin-based story is so frustrating. The MacGuffin spurs the plot; forgetting about the MacGuffin stalls the plot. Worse, it erases the value the characters put on the MacGuffin, which undercuts the realism of the story and, at times, can make the characters seem not fully invested in whatever their mission is. The only thing that should divert the characters from this mission is something even more urgent that, in the best case scenario, in some way will end up relating to or enhancing the plot.
Worse than stalling, however, is adding more MacGuffins before the first ones are dealt with. Again, we can look at Deathly Hallows. In each of the first six books of the series, a MacGuffin of one sort or another is introduced and then found by the end of the book. Book six, along with establishing and then revealing the identity of the Half-Blood Prince, also introduces the concept of horcruxes. Ah, thinks the audience, so these will be the five MacGuffins that Harry is looking for in the last book (as two of the seven have already been destroyed). But alas! In the final volume, we are introduced to three extra MacGuffins: the Deathly Hallows, in the form of Harry's invisibility cloak, the stone in Marvolo's ring, and the most MacGuffiny of all, the Elder Wand. Despite the fact that the horcruxes are a rather pressing matter, much story time is spent discussing the Deathly Hallows, their origin, Dumbledore's and Grindlewald's connection to them, the fact that the ring shows you wraiths that entice you to throw your life away, how Voldemort wants the Elder Wand but Harry's actually the master of it, and other things which completely divert from the actual plot of the book.
I'll come right out and say it: nothing about the Deathly Hallows actually adds to the story. In fact, I argue that they detracts from the message of the series as a whole. Is love the ultimate weapon against evil? Nope, you just need to become master of the Elder Wand! Is a focus on blood lineage a bunch of hokum? Not at all, because Harry and Voldemort are secretly related to the three brothers! But I digress once again.
The main reason Rowling should not have introduced these three MacGuffins was that she already had five, which get left by the wayside due to so much focus on the Hallows. Let me rephrase that: the reason she should not have introduced this new plot line was she already had a complicated plot line that gets diverted by this new addition. Chapters that could be used by our heroes to look for horcruxes are spent focusing on who is currently in charge of the Elder Wand. It's like the author is saying, "Yeah, I introduced a prophecy regarding Harry, and the idea that love is more powerful than magic, and that you have to slowly destroy Voldemort one piece at a time, but all that is boring. Here's a one-hit-one-kill-wonder weapon. Wouldn't you rather read about that?" Maybe I would, if that was what was introduced as the means of ending Voldemort. But it wasn't. Horcruxes were. Focus, Rowling, focus!
Lest you think I'm only picking on Harry Potter (I really do like the series, I promise!), lets take a look at another story with far too many MacGuffins: RWBY. The first several seasons of the show were pretty low-key fantasy school type plots, with magical fighting thrown in. We were introduced to our first set of MacGuffins in season three, in the form of the four maidens: Spring, Winter, Summer, and Fall. At the end of the season, the spirit of the Fall Maiden was transferred to a new host, leaving us to assume that in subsequent seasons we might be introduced to the other three maidens, who the good guys and the bad guys want for... reasons? It's a little vague, but still intriguing. We meet one of the other maidens in season five and it looks like we might be on our way to—Oh, just kidding; she leaves the show and our heroes and villains have a new MacGuffin that they want: four relics that are housed under the four schools. So they coincide with the maidens? No? O-okay, well, I guess in the next seasons, they'll be looking for the maidens and the relics, and—Nope! Gods! There are two Manichean gods who destroyed the world and will destroy it again if the relics are brought together and humanity is still fighting, so that's why the good guys are gathering them... during a war? What? Wait, where are the maidens? Where is the plot? What's happening? A lot of viewers jumped ship at this point, and although I'm going to stick with the show (I have a high nonsense threshold), I don't blame them. It's gone off the rails. We don't know what the characters want, other than the relics, but we don't really know why they want them, since, as long as the relics aren't brought together, everything is hunky dory. Truly, truly, I do not know what the writers were thinking introducing so many plot lines. The Maiden MacGuffins were driving the plot, but by introducing the Relic MacGuffins, it necessarily diverts the plot elsewhere.
One final clarification: I'm not saying that a story can't have multiple plot lines. I love stories where each character wants something different, and thus leads the narrative in different directions. What I'm against is letting any of those storylines die on the vine, which is what happens when MacGuffins are forgotten and characters either sit around not going after them or run off after new ones. This makes the audience assume that the MacGuffin isn't important to the characters, that it has no intrinsic or narrative value, and that it doesn't matter. What writers need to ask themselves is, if it's that unimportant to characters and doesn't drive the plot, why include it to begin with?