It seems most players who dislike Titan Quest decry it for what it isn't. Here's what it is...
Gameplay
Click-click-click. Titan Quest can be played with one hand on the mouse, and one hand lifting your favorite beverage. An occasional keyboard key is needed for revealing the drops, spellcasting, holding a range fighter still, etc. It is a very simple and accessible control system.
The animation, sound, and sense of control are improved in the retail version over the demo (at least on my setup). I had experienced mouse lag and noticed some sync issues that have seemingly been fixed, and I am very pleased with the feel and flow of the battles. The ragdoll physics which at first seemed out of place now feel like a true enhancement. Whack a beast, and you feel the power of your blow as it flies across the land, comic-book style. Titan Quest is very satisfying in that regard.
Graphics
Titan Quest is exquisitely detailed, particularly for an overhead perspective game. It definately feels next-gen in the graphical sense as obstructions fade beautifully when you've travelled behind them. The models are wonderfully crafted, particularly at the zoom level which suits the gameplay best, and minute details provide an engrossing environment in which to thrash the many enemies.
I have not had one single slowdown as some others have experienced. There's been no stuttering, draw-in, mouse lag, or any other technical problems that have detracted from the gameplay. I get some tearing, but it isn't a significant distraction, and I could enable V-sync if it was.
Sound
The sound is good quality. The grunts during battle are particularly important in a hack-and-slash game. Titan Quest grunts are fair to good, but not outstanding. The score is good, and the voice work is very good (save the "your inventory is full" guy, he is not good).
Value
I played several hours last night, and haven't made it 1/3 of the way through the first of three campaigns. A single run through Titan Quest will likely provide over 40 hours of enjoyment. Two more difficulty levels offer extended gameplay.
Titan Quest lacks randomized areas, and this has drawn ire to the hordes of D2 faithful, including myself. After I'd settled into playing Titan Quest a while, and started a second character, I found it didn't detract as much as I'd thought it would. After all, my favorite D2 areas were NOT randomized. For all the "Bloody Runs" I'd completed in D2, I should've known this already.
Tilt
I am having fun playing this game. The last game I played was Oblivion. I spent many hours playing that game, but found myself thinking it was taking many hours for me to walk my hero around the town. In the end, it just wasn't all that fun for me. Titan Quest has delivered visceral action where it was sorely needed - the hack-and-slash genre.
Demo
When I played the demo I was disappointed. I had some mouse lag and sync problems, and I felt the pace was slow. In the release title, the pace begins slow, but quickens. The more I played the full game last night, the more I wanted to continue playing. That is a sure sign of a good game, folks. I can't wait to play tonight. For a character builder like Titan Quest, perhaps the demo should've started at mid-level, so the game's content could be truly appreciated. I believe the demo was a misstep, and the release title is poorly represented by it.
Multiplayer and Cheating
The multiplayer browser is a Gamespy browser much like you'd expect to see on a FPS. I think it may be difficult to join good coop games unless you find some like-minded/skilled gamers on a good forum. The servers are open, so there will be cheaters. Player vs. Player is not part of the game, and one hopes cheating will be less prevalent because of it. I expect many will cheat, download "trainers" that suck the fun right out of the game, and spoil the fun of others. I strongly advise against doing this, because much of the fun is found in finding great items (not to mention executing untrusted applications is a great way to support Trojans). That said, even games with strong anti-cheating measures face farmers and people willing to pay real-world dollars to enhance their character. It's madness that we can't even enjoy our fun without a race to the finish.
Summary
Titan Quest is a fine game. I am recommending this game to all I know who enjoy gratuitous monster slaying. While some can't get over that it isn't Diablo 3, I submit it is an excellent title all its own, and worth the price of admission.