Fixes all the problems that I had with Diablo II and surpasses it.
I liked Diablo II, but I loved Titan Quest. Better inventory system, no body recovery, respecting, etc. The gameplay is very similar to Diablo II but the Spheres of mastery allow you to create a multitude of different character classes.
It does have a few flaws. It is possible to collect millions of gold that there isn't really anything to buy aside from potions and respecting. You can almost always find better gear in the world than at a vendor (although I did find a divine amulet that raises all skills by 1). Also the boundaries of the game world push believability. Here's a guy that can kill just about anything but can't climb over a few rocks.
I can't comment on the online play as I don't play anything on line because of all the griefers and jerks who make up a large percentage of the online gaming community.
Great game with solid game play and huge amounts of replayability.