I've done my best to put everything that can be considered a spoiler in a tag.
I've played this game through 4 times on Classic - mostly to experiment with builds and do all character quests lines. I am on my first Tactician run.
What I've noticed, in addition to the obvious mentioned things like increased monster numbers, are 
substantially increased number of traps and also items that are visible from a distance but disappear when you get close
.
For example, the Imp Traphouse in Cyseal.
In the room that has the 
two bookshelves with Lava everywhere
, there are 
6 keys -
 5 regular "Key"s and a "Chest Key"
 laying on those shelves that are visible from a distance, but magically disappear, with a sparkly  animation, when you get close to them. I can get close enough to 
loot all of them with telekinesis
, but can't because of the aforementioned behavior.
I know I can 
destroy all of the chests located in this area for their loots
, but does anybody know how to actually obtain these 
?