I would only say it's a hack and slash mainly because the NPC's don't respond to the environment eg. you can take anything that's lying around without being attacked by the NPC's or being jailed. I would really just generalize it as a dungeon crawler though it's not like bioware or bethesda where you're able to build relationships with anyone you choose or go with a faction, the story is linear except for the character development and the environment.
Now that I have started playing divine divinity I was surprised when I picked up something in someone's house and they threatened to call the guards. Guess that was dropped for DivII although I personally would have enjoyed that feature.
I have to disagree with you, I don't think you can tag this as hack-n-slash or dungeon crawler. You do build relationships with different characters through side quests, these relationships can result in lower trade prices, special items, or secret information. In many cases if you look hard enough or use mindreading these quests can be solved by more than one method providing more than one outcome (example:first Goblin Heart Collector or Two guys trapped in one body quest). There are even situations in the main questline where you have the choice to alter events (lord lovis' soul stone can be returned to him or smashed.) So I can't agree with your statement that storyline is linear, there is variation and choice with real results, the results may not be dramatic but they are satisfying.
I also should mention that I am a part of a faction, which was chosen during a quest I initiated while travelling too close to a bandit engagement with another traveling group. I had the choice to fight along either side with the little information I already had I choose the bandits. After helping the bandits clean up I was given the entry password for the bandit hideout, which upon investigation had many cool side quests and offered useful NPCs and unique weapons. There isn't a list of factions that you can choose sides with but there is a little bit of quest based faction alignment.
This game on the surface may appear to be a hack n slash dungeon crawler but it really isn't IMO, it is much deeper than that and much more satisfying.