In addition to setting the narrative path, your choice between Birthright and Conquest touches on a number of gameplay elements, including difficulty. Certain characters and classes will only be available depending on which faction you side with, like the splitting of the Thief class to be Ninjas for Hoshido and Outlaws for Nohr. It's fairly well spelled out in-game, but something you might not realize picking up the game off the shelf is how the game paths change the difficulty. Birthright is the easiest. Battles have more straightforward objectives that are easier for your leveled main character to power through and you can freely scout out new fights and challenges for experience and resources - aka grind. Conquest is a considerably harder challenge. You can't just grind your way to an overpowered team, and battles more frequently have unique win conditions and time restrictions.
While this helps to alleviate some difficulty swings found in previous Fire Emblem games, you should be forewarned if you were initially drawn to Conquest. Finally, note that Revelation is designed to be the middle point between the two in terms of challenge.
The gameplay itself will be instantly familiar to Fire Emblem fans, with a few minor differences. At the most basic level not much has changed in the series since its inception. During battle you'll take turns moving your characters around a square-boxed map and attacking enemies, which will then do the same in turn once you've moved everyone. Attacking an enemy pops up a window indicating your chances to hit, expected damage, and the same information for your enemies counter-attack before cutting away from the top down battlefield to play out the little fight cinematically.
Sometimes you'll feel like just skipping through them, but don't be surprised when you're glued to the screen if one of your beloved characters was accidentally out of position and needs to dodge an 88% hit to survive. Permadeath does make its return, if that's your jam, but you can elect instead to have characters be knocked-out for the rest of the fight or return on the next turn instead.
When you're not getting the plucky young villager you saved blasted with fireballs, there's plenty of satisfying tactical elements to exploit in your favor, like ranged units attacking melee ones outside of striking distance, and a lot of the basic gameplay revolves around this kind of positioning. The Pair Up system from Fire Emblem: Awakening makes a return, allowing you to group two units together to protect a weaker one, quickly ferry a slow unit around, or just combine into a single stronger unit.
The Pair Up system is also an easy way for characters to foster their Support relationships, little stories and dialogue that play out once characters reach certain levels with each other. This can lead to greater flexibility when advancing to new classes and romantic combinations may lead to children which unlock additional story chapters. There's also just something innately and strangely satisfying of deciding which of your quirky cast of characters are best for each other. It's like the Sims except love blooms on the battlefield not over taking out the trash.
The big new StreetPass feature for Fire Emblem: Fates is your personal customizable castle. The castle serves as your home base during the game, and you can wander around talking to the characters you've recruited. It's a big map area that you can lay out with gardens, statues, shops, and more. Heck, you can even change the overall type of castle along with the music that plays while you're there. There's even a little push and pull of wanting to line all your buildings up to be aesthetically pleasing or arranging them in a maximized order for defense, you will need to defend it from raiders at times. The one downside is that it obviously ate some resources from the world map. You're castle might be this customizable space filled with personal character, but navigating the world map is little more than a menu - which sucks some of the adventure and scope out of the experience.
Ultimately this mirrors many of the issues with Fire Emblem: Fates. While it might expand or maintain many of the best parts of Fire Emblem, it's not without a few flaws that mar an otherwise excellent RPG.
Bottom Line: Fire Emblem: Fates takes two steps forward and one step back for the series. It still retains and expands on many of the great Fire Emblem elements, but Fates slips on a few important aspects.
Recommendation: Dedicated RPG and Fire Emblem fans will find plenty to love, but if you're not fully invested you might be turned off on spending $80 for the whole experience.