Not too sure myself
I sort of liked Darksiders I, even though it was kind of linear and eventually started to feel a bit repetitive. But the whole idea and story was kind of intriguing nonetheless. I eventually started to lose interest a bit when all of a sudden the game went from 'all fighting' to 'all puzzling' which felt a bit out of place. After a pause I picked up where I left off and notice that some fights also became a little more tactical, which was fun in itself (like first getting portals opened up, then using said portals to move behind a boss and get him).
But I'm not too sure of number II. The story starts to feel a bit enforced to me. I mean; you're playing Death; a biblical figure which is supposed to be pretty much invincible. Yet you need to build up your powers (makes one wonder how he ever would have been able to perform the Apocalypse) and most of all; you can die? The situation somewhat made sense with War given that he was accused and bereft of his powers. But here we're talking about a powerful being coming straight from his normal place of residence. Who still needs to "level up" and all, which somewhat dents the idea of dealing with one of the powerful riders of the apocalypse.
Don't get me wrong; I can see it has all the potential of being a great game by itself. But apart from gameplay I also like the storyline to make at least some kind of sense.
A bit comparable to Fallout for example. In both games (3 & New Vegas) you also start off with hardly anything and need to build up, but it gets explained through the storyline. In the first one you actually need to build up your life, in the second game you've just been shot and need to go from a simple courier job to a wasteland survivalist. It adds up more IMO.
Some games such as Infamous go even further; in the first game you start from scratch. In the second game you already have most of your powers and need to extend to them. That makes a lot more sense storywise than "ok, version 2: time to start all over again".