It's not solely about the access to conveniently placed vents, though. The reason DXHR was sub par compared to its predecessor wasn't because vents were everywhere, but because it suffered too much from "choose between path A or B" syndrome, to the detriment of what real emergent gameplay. Dishonored did not fall into this trap.
Like I said, I think people are exaggerating this. I'm seeing it as an inevitable outcome of the game's levels being more cramped than the original DX's (Invisible War had even smaller levels and, what a surprise, was even worse about it), not as some intentional design decision. When everything is smaller, of course the paths are going to be more obvious.
But again, Dishonored avoids falling into the trap by simply removing from its game design anything that could create said trap in the first place. You can't have "paths" when your protagonist can just teleport anywhere he wants, so the game HAS to do something else entirely. Which is, I guess, turn you into a superpowered badass who can teleport through entire levels in seconds, but can kill enemies in various cool ways if he feels like it.
What I'm saying is that it's easy to be "emergent" when you've largely given up on the notion of physical obstacles from the get-go.
