Try and explain to the average user why URLs on HN look like this:
news.ycombinator.com/?id=123123
Whereas on CNN they look like this: http://edition.cnn.com/2014/05/04/world/africa/nigeria-abduc...
Whereas on another news site (Israeli) they look like this: http://www.ynet.co.il/articles/0,7340,L-4516118,00.html
Whereas on Reddit they look like this:
http://www.reddit.com/r/pics/comments/24p3tm/japanese_photog...
Each one of these is a completely different implementation detail which the average user doesn't care about and, honestly, won't necessarily understand without understanding the underlying technology behind these sites.
Remember: Most users barely understand, if at all, what a browser is! And if you want to see a comparable challenge, try to explain to someone just one thing - why do some sites have www vs. not.
The parent's sentence was really great:
"You underestimate how subtle the concepts underlying addressing are, probably because, like many technical people, you have understood how URLs work for so long that you can no longer remember what it is like to not understand them."