Can't see it so far in this thread, but this comes up pretty regularly.
Here's the relevant FAQ link; Gurus
In short, the 100 people who play the most games of Catan each month are the Gurus for Catan.
Each season they get to suggest new rule sets, selected from the standard options you see when creating a Catan table.
Those suggests can be up/down voted by the same set of people.
Which ever option has the highest overall +ve vote (Ups - downs), gets put in a head to head vote against the current format.
Whichever option wins that vote becomes the new arena format.
As mentioned in multiple threads this means gurus are simply chosen by volume of play a simple count of games, not time, not wins, nothing else, you don't even need 100 ELO (based on my experince with some v.small player base games). So a lot of games (and Catan is a poster child because of a large player base) default to settings that lead to quick games. Because people who play quick games can play more games in the same time period.
