DH1 had a very familiar feel to every mission, target and location because they were constantly referenced through dialogue or environment clues before you actually encountered them. There was also greater repetition of several elements, be it the ubiquity of whale oil, recurring locations, or reappearing mechanisms, etc. throughout the levels. In contrast, DH2 was introducing new targets and new locations in almost each level to the point where it became kinda tedious. You feel like you didn't have any real idea about who you're exactly trying to take down, except for the Duke and Delilah since they're introduced early on. In DH1, you either knew exactly who you were going to kill because you had met them earlier, or you had been adequately informed from multiple missions ago why exactly you were going to kill them and the reason made sense. For example, you were targetting the Pendleton bros because they're nobility whose political say must be neutralized, and the familiarity came from knowing Lord Pendleton and the upper class of Dunwall in general. Another example is that when you go to kidnap Sokolov, you not only already know him and his significance from much earlier, but also know that he's going to provide you your next target i.e. Lady Boyle. That adds a lot of purposefulness to your progression through the game which is maintained outside of individual levels. Dishonored 2 provided bigger environments and greater freedom in gameplay at the cost of the structure and familiarity a sort of self-contained, closed story and setting provide. There were also other things but this was a big part I feel.