I've been playing this game for ~15 years and it seems like every time I play it I find something in the game that I've never found before.
I'm only on Chapter 2 for this run and so far I've found 2 things that I've never seen before.
The first was during Korgan's quest to find the Book of Kaza. During the quest you visit a crypt to search for the book, and it's full of typical tomb-y things: urns, coffins, mummies, undead, sarcophagi, etc... Well, down on of the halls you can explore there is a rather inconspicuous looking sarcophagus leaning against a wall that just looks like any other random piece of set design to help set the atmosphere of a crypt. Howerver, I happened to mouse over this during combat and noticed that my cursor changed to the interact-with-circle-arrows icon. Clicked it, out popped a skeleton warrior, it was pissed off, i killed it, and it dropped a magical spear that I had never encountered before.
The second was during my liberation of De'arnise keep. On the 2nd floor you run into Glaicas who was a member of the house guard, however is now hostile. I assumed that he was a traitor and killed him ever time I had encountered previously. Yet, this time I noticed that he said that he was shown the "truth" by the enemy "sorcerers", and I thought "Hmmm sorcerers...could be charmed." So I threw a Dispel Magic before I bashed his face in just to check. Sure enough! His circle changed from red to blue and the dialogue window pops up! He explains a little bit about what's been going on, hands you the head to the Flail of Ages he normally dropped when killed, and even tells you about the hidden forge. For taking this path rather than slaughtering him on sight I was rewarded with over twice the experience I would have gotten from just killing him.
So! What sort of new things have you found in the game for the first time after your 2nd, 3rd, 4th....n'th time through the game?