Calwings posted... A few more points to add to this:
* Emblem rings can give characters weapon options that they normally can't use in their current classes, which opens up new ways to use them in battle. Example: Alcryst, an archer, can wield a Rapier when engaged with Marth's emblem ring, or a Killer Axe while engaged with Leif's Emblem ring, despite archers not normally being able to use swords or axes at all.
* Weapon forging is more in-depth than in past games and weapons can be customized in plenty of different ways.
* Story stages can no longer be cheesed with 1-2 overleveled characters or by quickly killing a boss with warp shenanigans due to enemies having chain attacks/blocks and bosses having revival rings that essentially give them multiple "stages" to defeat them. The game forces you to build a good and balanced team and think hard about your team composition.
* There's a good amount of side content outside of the story, but it's optional and has little effect on the core gameplay, so you have the freedom to explore as much or as little of it as you want. This is in stark contrast to Awakening/Fates, where you basically had to go through a glorified eugenics simulator to set up your support conversations and recruit/ optimize the second generation characters, and Three Houses, where the Persona rip-off social aspect wasted a lot of time and had a huge effect on your characters' performance in battle if you skipped or half-assed it.
So in other words, Engage drastically trimmed down the non-gameplay fluff and focused specifically on improving the core strategy gameplay to be the best experience that it could be. It's more challenging, more in-depth, and more (no pun intended) engaging than any FE game in recent memory.
Oh right. Giving new weapon access can drastically boost some units because they have control over the weapon triangle. Or at least can mitigate their weaknesses some.