There's various tuning but for me the main difference, but I could be wrong and not remember well previous books, is the significant quality increase in exploration design.
I haven't played a RPG with that quality in that field since a long time. This point is deeply linked to the quality of tricks/stuff to find/sort of secrets/sort of puzzles or riddles, not only there's many but also their difficulty is very well tuned, neither too obvious neither too difficult, excellent balance, at least for me.
In short the exploration is that great because:
- The amount of secrets, tricks to find, stuff mildly hidden or well hidden in a corner. That point is particularly improved vs previous books, if I remember well.
- The links between the maps and overall use of obstacles. Possibly using reveal map spell will break somehow this, but it's not the point. So it's great links with many "preview of a place... how reach this place" with path to find the secret not too obvious. It's also an excellent use of obstacles to build a wide areas where paths aren't too direct and obvious. That point is also quite more developed than in previous books if I remember well.
- Many secondary locations/path well enough hidden to require some attention and observation or a careful exploration. I think the new graphics helped improve this. The point is better than in previous books if I remember well.
- Overall not a single place look the same than all other in all areas, you'll find some exception but that's overall true, and that's a more general point there isn't a single filler with eventually two mid exceptions. That sort of constant diversity do help a lot the exploration fun. I feel Book 2 use a bit more fillers than the two other books, I could be wrong and not remember well.