My initial impressions are really good. It has everything I loved about the first 3DS entry in the series, Fire Emblem Awakening, but it's been refined and added to. The core of the combat system is still the same as is the relationship system and I'm really happy with that. It didn't need big changes. The minor tweaks they've done are for convenience.
The biggest difference I've noticed right from the start is the attention paid to the story. In Fire Emblem Awakening it had a very typical (and stereotypical) story that was easy to predict even though it was well executed. In Fire Emblem Fates they've created three unique storylines depending on a key choice you make near the beginning of the game. I'm already seeing huge ramifications and I can see that these storylines are all going to tie into one another in interesting ways.
It honestly has me excited to play through all three versions of the game.
For now, I'm happily playing Fire Emblem Fates: Birthright. I'll follow up with more in depth impressions or a full review later. If you've been on the fence about this game just know that it gets a rousing endorsement from me.