Boris Johnson's win is consistent with my theory that politics is a branch of the entertainment industry. Boris won because people liked him on TV, not because they had any confidence he'd do a good job. In fact, it simply doesn't matter whether he does a good job or not.
Whatever the budget of the GLA, the actual amount of cash he can shift from one activity to another over the next four years is probably on the order of only a few millions. He can change a few buses, approve a few "don't knife each other, there's good chaps" posters, approve or deny one or two large buildings.
On the other hand, he will be on television a lot, and get a lot more attention, because now he's (drum roll) In Government. And if you treat each of his appearances as a light entertainment programme, value it as equivalent to an equally entertaining non-political celebrity appearance, and multiply up the number of such appearances over the four years, his entertainment value to the voters easily outweighs whatever costs might be imposed on the voters if he is a Bad Mayor in a policy sense.
And in fact the predictable cost of Boris vs Ken is near enough zero. Who knows, Boris might even be better. While the predictable difference in entertainment value is huge - not only is Boris more entertaining than Ken on a level playing field, but more importantly the Ken show has run for eight years and we've seen all the best bits.
My point is that (a) Boris has been elected because he's funny and people are bored of Ken, and (b) This is, with apologies to Bryan Caplan, rational voting.
And of course, it is nothing new: Ken was elected in 2000 for just the same good reason.