Pleasant surprises outside the main mechanics (which have been much debated) included:
-the animals and birds
-tiny details on buildings and units that enrich the whole experience
-the unit camera
-multiplayer (which I have never tried before on a TW game, despite getting every one since Rome 1)
-the legion system (which I expected to hate)
-the province system (same as above)
-custom battle maps chosen on world map
-line of sight, and how it changes the battle experience (e.g. racing to be first to the top of a big hill)
-and most of all the massively improved ambushes, they've really got the atmosphere right on this one whereas in previous TW games it always felt a bit underwhelming, the attacker didn't have as significant an advantage as they should
I'm thinking of doing a 'hidden gems' thread with a series of more detailed posts on surprising/interesting things in game, like little impressive details or Easter eggs