Catan got really popular when I was in college, and I noticed a distinctly different playstyle among the "normal" groups of friends I had and the ones that came from the Economics department.
A lot of people never think to do things like:
- Pay the robber to rob someone else
- Ask the robber what card he wants to take from you, and if it's something you're willing to give up just show him which card it is in your hand
- Trade away all of a scarce resource you have, then use the monopoly card to get them back, then trade them away again
- Paying someone to build roads to cut someone else off
- Paying to "rent" someone's port for a turn to get a better exchange rate
We like to use a house rule that requires all trades to "clear" within a turn (ie no arguments over enforceable/conflicting trades taking place on future turns).