Unrivaled In The Genre
Diablo's "randomly generated levels" were just mathematical combinations of the same environments, not the finely crafted world of Titan Quest (Diablo II's Act III comes to mind, with its repetitive jungle environment, those little dart-blowers running around for what seemed like forever). Who needs supposed randomly generated levels when Titan Quest offers such a lengthy and diverse campaign (it took me 72 hours to complete, playing somewhat efficiently--that's with all the quests, exploration, and periods spent fussing with loot and vendors). By the time you complete Titan Quest and start on the next difficulty level, the whole sprawling world seems fresh again, no random generation needed.
In Titan Quest the rivers and waterfalls flow, the grasses and trees sway, and the villages and towns teem with jugglers and birds and tail-wagging dogs, while the sun rises and sets with the most realistic lighting I've seen in a game. The stirring music pulls you into the environments, which change seamlessly like in Dungeon Siege (another great game). Some of the set-pieces in Titan Quest are so well done (like the war zone in Act IV), that it feels like you're in a high-budget movie.
Of course, none of that means a thing unless the gameplay is good, and believe me, there is nothing to be disappointed about. It's utterly engrossing (for an action-rpg), and the combinations the myriad character options and equipment offer insure a very high level of replayability. The menus, inventory, and skill trees are smooth and unhindered, with too many streamlined and convenient details to mention. Oh, and in Diablo II, I only died once in the starting difficulty phase (facing Diablo himself), whereas in Titan Quest you really have to stay on your toes at the default difficulty, never knowing when something challenging might surprise you (as in a boss, a mob with a special ability, or that first cyclops--yikes!).
If you're into action-rpgs, you're in for a treat: Titan Quest is inspired, and unsurpassed in the genre. So much has been achieved in the genre over the years (the Sacred series is great too) that Diablo III would be hard-pressed to add anything new. But it wouldn't have to add anything new; all it would have to do is offer a meticulously crafted experience like Titan Quest Immortal Throne has.