Titan Quest begins with a familiar formula and progressively introduces a couple of interesting methods of playing the same adventure hack & slash in a way that is comfortable but interesting. If you've played entries in the Diablo series, you will immediately feel at home in Titan Quest.
This is not a bad thing, as Titan Quest does try to introduce some welcome game mechanics, such as the Mastery system, which allows you to travel up to two of eight different paths, creating 28 possible character "class" combinations.
You begin the game with a rather blank and boring character; you simply name a male or female character model--you will find no appearance customization options, and you will not choose your class during creation. How you set your character apart from everyone else will be determined by the choices you make while spending your skill points in the Mastery system as you progress, and how you choose to strengthen the 3.2 attribute scores that comprise your character's strengths and weaknesses. These scores are the 3 primary, Strength, Dexterity, Intelligence, and the 2 secondary Health and Mana.
The game varies in difficulty, within context of the available difficulty modes, from 1-hit hack & slash kills, to gruesome boss fights that will force you to run and regroup. Much like Diablo 2, you can introduce your character to increased difficulty modes to offer greater challenge and reward on subsequent times through the campaign.
Making your way through Titan Quest can go as slowly or as quickly as you'd like: you can set the pace. If you like to explore every corner of the world, there are secondary quests and non-essential lore in place for you. Conversely, if you like to fly straight from objective to objective, following the main story, the path is often clear, even despite the droves of monsters blocking it. While additional side quests and lore may satisfy some, it ultimately won't offer enough depth for fans of deep storyline RPGs such as Baldur's Gate or Planescape: Torment.
The multiplayer is seamless with up to 6 players--much like Diablo, the difficulty scales with each additional person. There are group-specific skills that allow you to bestow buffs, auras, or enchantments upon your friends and allies. The only notable thing missing from multiplayer is any form of player vs. player (PvP) combat, short of a special mode that must be engaged using a command line switch when the game is started--requiring each player to do the same--but is mostly unsupported and simply not very interesting.
Another notable feature missing from Titan Quest that made Diablo so interesting is the terrain/environment randomization, which Titan Quest lacks. Re-playability may become a little stale as you progress through the same areas time after time, but the game makes up for it by offering three difficulty modes that are only available by completing the previous--the drive here is finding those "ultra rare--rarer than rare" legendary (purple) items.
The bottom line: if you enjoyed games like Diablo, Sacred--maybe even Divine Divinity--and you're not tired of the genre, you will most undoubtedly enjoy a few times through Titan Quest to whet your appetite until the release of Diablo 3.