I was thinking yesterday of good examples of "show, don't tell" and one instance that stuck out in my mind comes from the second season of Buffy the Vampire Slayer. There's a particular instance where the writers wanted to make a particular point crystal clear to the audience and it feels like an entire two-parter was built around demonstrating that detail.
In the two-parter "Surprise/Innocence," Buffy and her friends are faced with a villain named The Judge, a demon with the power to burn the humanity out of anyone. Long dismembered, he's reconstituted and prepares to burn the citizens of Sunnydale, forcing Buffy to find a way to take him down.
Concurrent with this, Buffy and Angel have sex. A consequence of this is that Angel loses his soul and reverts to his evil, demonic personality of Angelus. It was a rather bold and shocking twist for the show, and likely creator Joss Whedon wanted to leave no doubt that the noble Angel was truly lost and that he wasn't pretending to be Angelus for part of a larger deception. Thus, fairly soon after Angel's reversion, he confronts The Judge, claiming to want to join forces. There's some skepticism on the Judge's part, perhaps fearing that Angelus might be a spy for the good guys - so he touches Angelus and the now-soulless vampre doesn't even so much as smoke.
"There's no humanity left in him," The Judge declares - clearly speaking for the writers. Soon after this, the Judge is summarily disposed of, having served the storytellers' needs.
I've read a lot of spec scripts that would have benefited from a similar approach. Too often - especially in mystical scripts, important details like "Angel is no longer ensouled. He's pure evil." might be covered purely in dialogue. The writer declares one character to be an expert in the mythology and then uses them to lay down all the rules and stakes the writer wants the audience to accept.
Whedon easily could have just shown us Angelus wrecking havoc and killing people, but even then there likely would have been some fans in denial that Angel had really gone over to the darkside. "Maybe he didn't kill that person. Maybe he's just putting on a show to convince the bad guys," they might say. (Believe me, hang out on the message boards for any show and you'll see a lot of this sort of speculation.)
By no means is this the only example of that sort of "showing, not telling," nor do I mean to say that it's the absolutely perfect example of such. I think it warrents mention because its one of the instances where I can think of a writer creating an entire mini-arc just for the express purpose of hammering home the stakes to an audience.
What are your favorite examples of such?