First of all, this game is inspired by Titan Quest. However, it is NOT Titan Quest, don't be fooled with the title. I will compare it with TQ time to time but basically just to show differences than similarities.
Is it good or bad? Neither. It is just another game. It inherits many features from TQ - the same engine (that's what makes some people to think the game is just a masked TQ), dual mastery system, basic skills ideas, itemization principles, inventory management and others. However, gameplay and overall feeling is not the same. I wonder why some people look at Grim Dawn as at skinned TQ because it feels very different in terms of pace, combat, quests, athmosphere. It is... err... faster. It's the best description I can imagine. Everything happens quite fast in the game. You start to feel your character powerful relatively fast - pretty since the first level. Many things support this feeling - from the actual raw power to hits animation which makes you feel impact on most of your hits. It could get boring soon if you would not unlock new skills with levels so you still will have something new to play with :)
Basically, it's the game about doomed and dying world where mankind was pretty much eradicated by two powerful magical races. You're one of few remnants survived the just happened Apocalypse and trying to establish themselves in the changed world. So there are lot of remaining ruines and lore in the world. I actually like the way Crate has placed world information and story in the game - it does not come in forms of long quest dialogs but as pieces of paper here and where which you can read if you are interested or skip if you are not interested in learning the story and want just to kill things. I tried both and must say the story is very well-written which is rare case for ARPG genre. Just try to read that diary of the dead child... The world is also well-detailed with many small pieces adding the feeling of doom pretty everywhere. Overall graphics is probably not that detailed and eye-catching as it was in TQ but I guess TQ had it somewhat excessive while GD looks just fine for the fast-paced game where you won't pay too much attention to details most of the time anyway. It is still look fine when you stop for a while and zoom in to see details like a doll on the stairs. So the graphics is just fine on my taste. It looks much better with AntiAliasing and AnyzotropicFiltering turned on though ;) Overall story and graphics is much more mature than TQ with some of dark humor and some serious emotions it causes but much less pathetics. Bad guys actions are basically better reasoned and logical based (if you bother to read the story pieces).
Quests. They are various enough to not get bored. You will meet those "kill X monsters" and "collect Y pieces" things along with exterminating big bad guys, securing areas (the last looks quite stupid with the monsters respawn but anyway), finding lost people and even uncover the treachery... or cover it. Yes, some quests seem to have several endings. Crate promises that you will meet consequences of your decisions later. Quests aren't necessary to finish the main story except first few ones which you will need to complete to get access to the stash. You can do even without them though - in my first playthrough I have got that access only after finishing the act as I just did not found the way to sewers to complete one of these quests :) Basically, if you have done what the quest requires already and then got to the quest giver, it will offer other dialog and accept the quest. Quite a reasonable system on my taste. You can just go forward and kill everything on your path and once you get to the end you will complete most of quests - just get back to the town and talk to people.
Advancing the game. GD has other ways to advance than TQ had. First of all, the game is not linear. You have a single base and move in various directions from it but still return there for supply and rest. Second, directions you move are not a narrow long path but quite wide areas where you can just move along the road skipping pretty much about 60-70% of content or go exploring. In the last case you will find many optional areas some of which offer significant rewards. So you are free to advance fast from riftgate to riftgate or stop in the every area until you find everything in it. Or you can advance fast and then return to the areas you've been already to explore them in details. The last way has the hidden benefit related to monsters level scaling. You can find new more dangerous monsters in the previously explored areas.
Monsters scaling. Monsters have limited scaling their levels with player in GD. So you can either advance to next area or just re-run the same one to find monstes have higher levels and you still get enough exp and better drops to advance in levels. Some people find it good, some blame the game for it. I personally think it is good because scaling is not infinite and if you run the same area for 3-4 times you usually will outlevel monsters there. So you still can feel yourself a stomping giant in you want :) Or you can just move forward as fast as you can - monsters will be either equal or slightly higher level than you. The system looks very well balanced and entertaining.
Monsters. What can I say... incredible. Especially considering this is only Alpha (close to Beta already) and the first act of the game. Monsters variety is amazing. You will find about 8-10 basic types of zombies and other Aether corruptions (almost every type includes 2-3 variations with distinct look and properties), the same amount of mutated animals types (again, with several distinct variations inside every type), two semi-inteligent races, two types of human enemies and two types of Chaos creatures (all of these type groups have several variations as well). There are lot of hero monsters (they exist for every monster type in the game) with unique abilities which offer much greater challenge with an increased reward (sweet loot :)). Believe me, combat never feels boring and you never will think "ahh, these zombies again..." - GD has almost infinite surprises to you :) Moreover, as I said above, if you return to the area you've been already when you get 3-4 levels you most possibly will find new monster types there. Some of these types are unique to particular areas and don't meet anywhere else in the game. So if you want to see all enemies be sure to revisit areas you've met every unique monster type when you get max level.
Combat. It is quite fast-paced but yet is quite controllable and leaves enough time to think your actions. Impact feeling is great due to various animations and screen shaking on crits. Crate has entertained new feature opposed to potions spamming - out of combat health regeneration (OCH). This feature is still questionnable and perceived differently by players. Some praise it while others blame. What does OCH mean? When you not get hit and don't hit for 2-3 seconds your health starts to regenerate rapidly and get to the max in 4-5 seconds. Some players are against it because think it can be abused and make the game too easy. Others argue that if you want to play reckless you can just continue hitting mobs instead of running around and avoiding hits. Anyway, this system really makes potions an emergency solution and rare used. Besides that, there are many ways to replenish your health during combat. I myself love this system and was upset when it was nerfed once already. However, we'll see what it will look like in the end of development. Comparing to TQ, combat looks ages ahead. Ah, I almost forgot about the new targeting system! Be sure to uncheck "Classic targeting" option in the game menu and try how it feels. I can't return to classic system now. The new one is pretty much the same you could get in Diablo 2 with right mouse button. Well, I could say more but seem to run out of space :)