Okay so the biggest most important change from E:wom to FE is that there is now a top quality designer behind the game - Derek Paxton (also Jon Schafer in support capacity).
A designer makes sure that
1) There are choices in the game that are interesting
2) The different parts of the game play well with each other
7) Appropriately difficult
9) Everything else
Perhaps the easiest way to understand this is to think of how attack and defense for units was handled in E:wom. There was one attack value, and one defense value. Those two stats were all that mattered in tactical combat. You always wanted more, more, more. And the more you got, the more unbalanced the game became. In essence, the mid- and late-game were broken.
In FE, there are three physical attack values, and four magical attack values. There are also 3 corresponding physical defense values, and four corresponding magical resistance values. They have different properties - just for example, blunt damage is large but slow, cutting damage is fast, pierce damage often goes through armor well. And then there's armor - chainmail protects well against cutting damage, plate protects against blunt. Monsters are often vulnerable or extra strong against a type as well (be it blunt damage or fire damage). That's just a brief overview, but right there you have 1) interesting choices 2) different parts that play well together 3) balance 4) pacing 8) UI is right.
So when we talk about E:wom -> FE, the important thing to understand is that EVERYTHING IS BETTER, and that's not an understatement.
Specifically, the most improved parts are
1) Tactical combat and the values that decide it
2) Heroes that level up with trait choices and magical items
3) Magic and its integration
4) The world around you with monsters and wildlands (and quests)
Beta 4 will focus on how cities and the economy functions, so we're all looking forward to that right now because it is not working great.
PS. There hasn't been any talk about the campaign, so I can't say if that's a selling point.