Settlers Variations

Friday's gaming activities focused on different rule variations of Settlers of Catan.  Our group played the four person team game, consisting of two teams of two players.  Though all of the players at my table had experience playing the game, there was a large gap in skill between the most experienced and least experienced players.  I thought the idea of the variation was pretty neat, but there were a few problems we encountered while playing.  Firstly - and this may have been due simply to our individual game - it took far too long for either team to reach the winning amount of points.  Secondly, the inability of team members to trade with one another made the game a lot more cut-throat and therefore generated an air of hostility throughout the game.  The trading rule also eliminated almost all trading during the game.  I would like to alter the rules to allow inter-partner trading.

Question: What is the most difficult aspect of designing a game?  Are certain parts, like board design, strategy, or game play more difficult to design than others?