At three hours, I've played the tutorial, a bit of the campaign mode, and a little bit of a regular game. The big thing I'm noticing is that combat got a serious upgrade in the expansion. In Fallen Enchantress: Legendary Heroes your different combat units got special abilities based on their equipment, making unit composition and placement much more important. Also, the way you level your champion units is more robust in the expansion, which only makes sense.
However, the core of the two games is essentially identical. It's got the typical 4X trappings - cities gather resources from the countryside and build improvements to increase those resource yields, becoming more powerful as the population increases and allowing you to build improvements and units faster while you research technologies to give you ever more improvements and units to build. But where Fallen Enchantress differs from most 4X games is in its focus on champion units.
Champions are unique characters with individual names and backstories who wear equipment you find in ruins (or buy from stores) and advance along an rpg-progression. These Champions are the backbone of a turn-based tactical combat minigame (well, technically, your generic units are more powerful in the middle-game, once your starting champion gear is obsoleted but before you start getting end-game gear and spells that leaves them in the dust) where you fight a variety of fantasy monsters in order to gain experience and loot.
Other games have done something similar (notably Endless Legend, which is a better game overall), but Fallen Enchantress really does have a knack for making its champions feel like significant characters. Exploring the map, completing quests, and gaining treasure is like an rpg inside the larger 4X. It's pretty satisfying, especially when you go for the epic quest victory (although this particular observation comes from my memories of Fallen Enchantress: Legendary Heroes and not from my current playthrough - this could get awkward).
I probably won't go back to the campaign mode, just because the normal mode goes by pretty fast and I'm eager to get to Legendary Heroes so I can play the game for real (sorry, original version of Fallen Enchantress).