Great game concepts are ones that seem obvious in retrospect. Here are some things I am really loving!
THE LOOT. I love this. Standard RPG stuff in a strategy game. I love it when I find a rare item. So cool!
THE PERKS. Standard RPG fare but made to feel natural in a strategy game. HOMM has something like this in FE it feels more natural. Plus, the AI seems to use it well.
QUESTS THAT CHANGE THE GAME. The Quests in WOM were lame. I love getting quests that end up giving me a special unit. More please!
THE MONSTERS. My first game I got killed by monsters and worried about the balance. Now I understand them better. They aren't random. Don't build a city next to one and stay away from them if you can't kill them.
THE SPELLS. Usually Spells are an RPG thing. Here the spells feel like tools of a strategy game. I love that you only want to imbue heroes that have the spells you want to access. It's simple and fun.
OVERALL I am just loving the way features usually thought of a role playing are rethought out to be in a strategy game. They didn't try to make FE a strategy/RPG. They made it a strategy game and rethought how features in RPG could be done in a strategy game. That's the biggest change in FE over WOM is they made decisions and focused on those decisions instead of tried to do everything [badly]. That's good game design. Making tough design choices. Good Job Stardock/Kael