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

3) Balance

4) Pacing

5) Lore

6) Fun

7) Appropriately difficult

8) UI

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.

The strengths and vulnerabilities can be quite important, but no physical attack is useless. Immunity to a certain magical damage is more common.

Another factor is the differentiation between the factions.  They are very different now and how abilities that emphasize the different ways to play the game (special units, unique spells, weaknesses & strengths).   The world is more dangerous and alive, and there are new wildlands that are really challenging areas with boss monsters.   The AI has gotten a lot better as well.   We'll see in the next beta version (beta 4) what changes they make to the city building.

The first thing is that it isn't an expansion its more like a sequel.  Also, you probably know this already but depending on when you bought WOM you may get a discount or even already qualify for the beta.