It's a completely dice-less game, which means that you as a player can't rely on your luck to defeat an opponent. It also means that the game has plenty of advice on how to make things interesting without the randomness of plastic platonic solids. For instance, if the players discovers that a certain enemy is to tough for them, then they are BOUND to come up with a way to tip the battle in their favour. Player creativity, not luck of the dice, becomes the winning factor.
Theatrix also added in a strong degree of player created content and player meta-input (that is, the players could, with the help of Drama Points (I think it was called) rewrite/add facts into the scenes and the story). The book also had a couple of interesting examples of how to do this
Finally, the game contained a very nice introduction to the three-part story-structure, which has helped me quite a lot in my game mastering (even though I'm drifting more and more towards situational gaming nowadays).
So, if you find it, pick it up. While it may not be the best game out there, it is definitely one of the better Gamemasters Guides.