Ah Post-Apocalypse how I love you. Since the heady days of Gamma World, into more terse Fallout fan creations, to more modern Other Dust it remains one of my most favorite genre's to play in. Part of the reason is I enjoy urban exploration. Digging through urban ruins and the decayed ruins of what we consider modern and clean.
Dead Malls, a great site for closed and decaying Malls. One of the tropes for Exalted is Lunar and Solars righting over the decaying runs of 1st Age Shopping Malls and that idea stuck in my head. Especially when you consider the mall could be something like:
The world's largest mall over 98% vacant. How awesome would that be?
One of the biggest Airports made, but hardly used! It's amazing and creepy.
Urban decay and the slow recline of civilization is part of the wonder of these games. Look at pictures of decaying Detroit and you get the idea. It's not all about just the decay, it's making something out of the decay. I honestly enjoy the Fallout series of games more than Skyrim. Mostly because even though the sewer X and ruined building Y do get repetitive, I love scavenging in games. Picking up whatever I can to make weapons and gear needed to survive. I wish more RPGs played up this aspect of making the most of what you find.
Another is how cultures change. History distorts and eventually either through mutation, adaptation, or culture push humanity or whatever race you are playing changes. Eventually you get into trans-human, post-human, or post Apocalypse culture concepts. Alien societies and worlds develop from the ruins. I find the rebuild what we had boring in turn. If the world has to end, change something!
That said, it's also a great group motivator in games. Survival depends on teamwork, and survival when the world can kill or change you overnight is very hard and takes more critical thinking and buddy support than most games. I've found you can tell a lot about the quality of a players commitment to a group by how they play characters in these settings. Good players tend to be helpful, as long as it's not a plot element for them to betray everyone, while problem players tend to horde and let the rest of the group die at the hands of whatever glowing thing crawled up on them this week.
In the end, part of what makes the post-apocalypse genre fun is it's what you make of it. It can be psionic new society adventure, robotic nightmare fuel, strange new mutant land, bizarre body horror fetish, the hard press for a new earth, or Planet of the Apes.
Either way, we had to blow it up. BOOM!