OK, so Titan Quest does improve on Diablo II in some areas. There are more towns and villages, and more of a sense of exploring a world, not just popping from dungeon to town to dungeon to town until all the monsters in the area have been eliminated. Titan Quest also plays smart in its portal system, which keeps you zipping from area to area at speed and without penalty, and in the way it treats death: you restart at the nearest checkpoint with health and energy maxed out, all possessions intact, and your only major problem being the hike from there to wherever you met your not-quite-end. Titan Quest also does some nice stuff with pets and summoned spirits once your skills reach a certain point. While you can use the Ctrl key to select targets there's no need to micro-manage them – they'll happily attack anything in sight with roughly the same amount of thought that you'll exhibit.
Get the basic idea? Now for the second point – does it matter? Strangely, for the most part, no. On paper, Diablo, Diablo II and now Titan Quest may sound boring, but in practice they feed our compulsive urge to batter beasties, level up, acquire new items and batter new beasties to death. It's a kind of vicious circle; first you're unwilling to stop because you're just about to level up, then because you've just found or bought a new weapon and you want to see how much damage it can do, and then because you've just opened up a new area and what you want to see what or whom lurks within (and how soft its skull is). And just when you achieve one aim, another one turns up.
During my many hours of playing Titan Quest, my conscious mind was constantly informing me that it was repetitive and slightly boring, but every time it was too late: my unconscious mind had taken over, and was leading me towards the next monster lair in search of treasure. Titan Quest is the kind of game that drains evenings and weekends one hour at a time, and when you come away you'll wonder why. But then, when you're weak, that insidious idea enters your brain once again; why not fire up Titan Quest? Just a quick game, what's the harm?