This was the most useful (writing-related) panel that I watched. Although it was talking in specific about real, existant London, there were lots of excellent worldbuilding nuggets to take away.
London in Fact and Fiction
One effect of the Blitz is that there remain Tudor-era buildings beside the most modern of structures. Construction from radically different time periods is side-by-side because of the patchy destruction caused by the bombs--keep this kind of effect in mind for world-building.
Clean-looking cement still exists in some places. The technique for making it has been forgotten since.
Often, old ruins (previous history) are discovered and quickly excavated before they have to be covered back up again so that the city can keep on growing.
Historically, "The X Arms" is the pub you'd go to in order to meet people from X profession. Bricklayers, etc.
The first city to invent or implement a new thing is the city that has all the errors and bugs. For example, the London Underground only has one track, so they have to shut it down every night to go in and clean, instead of letting it run continuously. (The people responsible for cleaning out the hair from the Underground to keep it from catching fire are called fluffers, by the way!)
Secret London: http://www.secret-london.co.uk/Welcome.html
Great discoveries are made in places that are horrible to live in. For example, the cause of cholera was discovered because of crowding and water pollution.
London has laws requiring the keeping/presenting of a historical object in public view despite it existing in a commercial space.
If you're writing something set in a foreign city, try having the PoV be a non-native to help cover for errors.
All CONvergence 2013 posts: http://cloudscudding.livejournal.com/tag/convergence%202013