Plot Flaws: What's a MacGuffin?

This plot flaw is new to me, but it's a good one.

So what's a MacGuffin?

BTW: Alfred Hitchcock apparently coined this term with a great anecdote about Scottish lions.  Here's an essay by Michael Kurland that starts with this story:
The Writer's Toolbox

Far as I can tell, it's an unexplained plot device that catalyzes action.  MacGuffins drive the central movement of the story, but the driving goal or object isn't necessarily explained.

Funny, most of the examples I have run across have some explanations and meaning, but the main point of the MacGuffin is that the explanation either doesn't matter, is meaningless, or is superficial.

Let's look at a couple:

1. Rosebud.  The last words of Citizen Kane drive the entire plot of this movie.  Funny thing about this movie is that the basic implication (spoiler alert! but then again, if you haven't seen this movie, a curse on you) is that only the audience knows what "rosebud" means.  No one else in the movie will understand what this means.  By the way, what did Kane really mean?
2. Hamlet's father's ghost.  The words of the ghost drive Hamlet's actions and the plot for the play, but in the end, the ghost is right.  The tension in the play is watching how Hamlet comes to understand everything.  I wonder what this play would look like if the ghost had been wrong....
3. The Maltese Falcon.  This one keeps coming up.  The basic premise here is that a really valuable thing is driving the action of the plot, but what the item is, doesn't really matter.  In the end, greed drives the plot.

Mostly, I'm loosely calling this a plot flaw because I think that the professionals can work this extraordinarily well.  If you don't know what you're doing, a MacGuffin may just look foolish, like your characters are chasing after butterflies.