To sum up Grim Dawn, imagine Diablo 2 crossed with Titans Quest, with a tiny hint of roguelike thrown in.
This game is being developed by some of the developers from the creators of Titans Quest, and is using the Titans Quest engine. However, within the engine they have managed to change the game enough to make it distinct from a Titans Quest mod. Think similarly to Unreal Engine. It may not be as flexible, but god damn if Crate can't bend it anyway.
The game heavily favors exploration, with secret quests, bosses and chests hidden around the world. Its fun to find new stuff, and even more entertaining to blow a friends mind by showing them where a level 30 boss is in a level 18 zone. Yes, he will destroy you if you aren't very prepared.
There is very little hand holding inherant in the game, however the community more than makes up for it. Due to a previously successful kickstarter, there is a large community who have been helping guide newbies, and also to just have fun together. I would highly recommend joining the forums, and atleast reading the developer journals 'Grim Misadventures' as they provide sneak peaks at upcoming content.
Yes, this game is incomplete currently, however, what they have released so far is well worth the price in my opinion. Torchlight 2 was a similar price, and may currently have Grim Dawn on play length (I think it would actually be close however) but only up to Act 3 Chapter 1 is currently playable.
The build system will be similar to anyone who knows something about Titans Quest, with the combination of two different skill trees, i.e. Demolitionist+Nightblade=Saboteur.
Simply put, if you enjoyed older ARPG's, filled with secrets and crazy builds, then you will most likely enjoy this. Atleast give the forums a peek.