Good
As the title would imply, the game takes place in the ancient world, and before you're through, you'll have traveled through many centers of civilization, including Greece, Egypt, China, and Babylon. The impetus for the story suits the game's core competency (i.e. killing hordes of monsters) just fine: the Titans have gone crazy, and thus have driven most of the world's non-human inhabitants insane. In the role of "hero," it's your job, of course, to destroy them.
Your beginnings in Titan Quest are humble enough. You don't even have a class when you start out -- you simply choose a name, gender, and tunic color, and off you go, club in hand. It isn't until you reach level two that you can choose a "Mastery," which is Titan Quest's answer to classes. There are eight in all: Warfare, Defense, Rogue, Hunting, Nature, Spirit, Earth, and Storm. The first four mentioned focus on direct, physical combat (or bow use, in the case of Hunting), while the last four are more magical in nature (Nature being focused towards healing and support, Spirit conferring necromancer-style powers, and Earth and Storm dealing with direct damage of the elemental variety).
Each Mastery is essentially a skill tree that you use to develop your character's capabilities in battle, and each one has enough skills in it to allow you to experiment with different applications of the same base concept. You get a few points to plug into them every time you level, and scattered around the world are "seer" NPCs that allow you to undo your choices for increasingly steep fees. You can also choose a second Mastery once you reach level eight, which allows you to further specialize your character. Similarly, every level you gain grants you a few points to allocate to your character's base attributes. Overall, Iron Lore has done a fine job of balancing player choice with concrete, defined roles. All of the combinations I've tinkered with are very engaging in their own right, and they all have suitably unique feels.
Well, "unique" for the purposes of a click 'n slash RPG in any case. There's no way around it: Titan Quest, like its source material, is all about clicking on monsters until they die and seeing what flies out of their bodies afterwards. To its credit, Titan Quest does a very good job of putting you in situations in which you can do just this. Indeed, there's very little to get in your way if you're determined to do nothing but this.
