The basic idea of Thurn and Taxis is that you are building postal routes to build a complete postal system. As you build up the various bits and pieces you receive victory points. While some objectives are the same for everyone, most objectives are worth more if you are the first to accomplish the goal. Some goals you may accomplish more than once.
One of the striking things is how little player interaction there is in Thurn and Taxis. It feels largely like each player is playing a form of solitaire. There is little you can do to impact another player and in those places where you can, the cost of doing so would likely be so high it would cost you too dearly to do so. Arguably you are more likely to be hurt by bad luck than you are to be hurt by an opponent.
In that sense, the game feels more like a race. The first one to accomplish a goal receives a slightly higher rewarded that he/she who finishes second, third, etc. In addition, the bigger the goal, the bigger the point. Your paths to winning are, therefore, going to be to either accumulate as many rewards as you can, or alternative to accumulate fewer larger rewards.
The player most likely to win is the one who has to rely on luck the least. There are moves in Thurn and Taxis that are clearly inferiour to other moves. In T&T, you must build on your route each turn. If can not do so, you are forced to scrap your route, meaning you have effectively lost most of the value of your previous turns spent building the route. Route scrapping will inevitably lead to losing (as you just aren't winning the races).
In addition, you have 4 special actions you may chose from on each turn. Two of these are clearly the best actions (drawing two cards or playing two cards). The other includes the ability to treat a route of size <n>
In the final analysis, Thurn and Taxis is a good social game. There need not be a lot of downtime between most turns and the inability to conduct open warfare on your opponent will appeal to many social gamers. I suspect however that the game has limited appeal to hardcore aggressive gamers.