The 2nd major expansion to the original Carcassonne, Traders & Builders aims to shake up the original play just a little more than Inns & Cathedrals. The term Traders in the title, weaves its way into the game by introducing resources to the world of Carcassonne. A total of 20 city tiles are offered and each of these depicts a resource symbol - wine barrel, grain or cloth. These are placed to create cities just like any other tile but they offer a special bonus. If any player (please note the underline) can place the final tile to complete a city containing resources, they will earn a resource token to match each of those resources. At the end of the game a bonus 10 points is awarded to the player that holds the most tokens of each resource. That is a whopping 30 bonus points that must be contested as the game progresses. The obvious strategy dilemma here is that players may feel compelled to complete an opponent's city, earning them points, in order to earn resource tokens and have a shot at the bonus points. Diabolical!
The 2nd shake-up takes the form of the builder and pig units. These both come in the form of additional wooden pieces and are added to your follower pool at the start of the game. The builder is an extremely powerful piece. It can be placed by a player whenever they add a tile to a feature (city or road) that they already control. If in future turns they can add a tile to a feature that contains their builder they will earn a bonus turn, which is taken immediately. This is huge as a player can effectively increase the value of their city or road and earn a bonus turn into the bargain. Only one bonus turn can be taken each go. If a feature is completed, the builder and all followers are returned as per usual.
The pig is placed in a similar fashion to the builder but this time it can be placed on a farm region that the active player already controls. It stays in play for the entire game and if that farm region earns points at the end of the game, it will earn 5 points per completed city instead of the usual four.
This expansion also offers a bonus, this time in the form of a cloth bag. This is not only nice but almost essential as the number of tiles used in the game (if both expansions are used) is getting high.

In terms of game changing mechanics, Traders & Builders is the more dynamic of the first 2 expansions. But like most things it will come down to personal taste. This is my favourite of all the expansions as the thinking required is increased substantially. However other players may feel they now have too much to think about, especially if they are also using Inns & Cathedrals with this expansion. The only true way to know is to play it for yourself and I highly recommend that you do!