What aspects of Titan Quest have been improved for Grim Dawn in your own opinion?
The biggest are mood / atmosphere and combat. I always felt like TQ was a little flat in terms of the atmosphere. There wasn't that sense of mystery and danger that the Diablo games do so well. We've put a lot of effort into developing that more and creating an interesting world with a well-developed history. We've also put a ton of effort into combat and pacing. We've added blood / gore effects, enemy hit reactions, special deaths for some enemies, others ragdoll, zombies can be dismembered, hit sounds are crunchier, blood spatters on the ground and pools under bodies, etc.
There are also a lot of cool new features though. The more open world is something I really enjoy as a player who prefers to ignore the main story and run off exploring. We've also added more randomized elements, dynamic weather, a faction system, destructible environments - really cool busting through doors or blowing up walls to reveal hidden areas.