To me, the most unforgivable trait in any public person is the lack of a sense of humor. People who can't find a way to laugh at the circumstances they find themselves in, or make fun of themselves, are in danger of losing perspective and over-reacting, or becoming obsessed with something trivial. In short, a sense of humor is a sign that you can see the forest for the trees.
That's more true of mayors than most other public posts. Mayors, by the nature of the job, are at the retail level of politics. Their job is about tangible things (potholes and garbage pickup), and their constituents are close at hand (you can't run off to serve as mayor in Washington).
So, today, one of the great cities in the world, London, is choosing between a dour, but liberal, former mayor in Ken Livingstone, and a fun, but conservative, current mayor in Boris Johnson. Livingstone's politics probably match his city better, but people just like Boris and his antics.
For example, Johnson is known for his love of cycling, and regularly bikes to work. (The bike sharing system in London is commonly known as "Boris Bikes," which must be a great disappointment to Barclay's, which actually sponsors the bikes.) He has been the victim of several bike thefts, and has expressed his desire to plant "decoy bicycles throughout Islington and send Navy SEALs in through the windows of thieves."
As things stand right now, Boris is six points ahead in the polls. It looks like Londoners might pick wit over ideology. I'm not sure that's a bad thing.