F4LL0UT: I don't know who had the idea to turn the game into an offline World of Warcraft
Oh come on, that comparison is just unfair. The character design might seem a bit similar, but that's because both Darksiders games and WoW go for this cartoony aesthetic. Aside from that, the world is very small and tight, navigating it only takes short periods of time (all of the worlds to be more accurate), and it takes the metroid-ish elements to a whole new level with optional dungeons - which also adds a new layer to exploration.
Aside from that, combat system is just as far from WoW as you can humanly get, being far, far more hack and slash than an RPG, as well as all the combat and some non-combat areas having a lot of hack-and-slash like platforming, separating it from a typical RPG even further.
As for the character of Death, it would feel weird if he was just a more badass War - as it stands, I felt he had much more actual personality than War ever had. Death is sneaky, stone-cold and cynical. Just as I'd imagine death to be.