Amazing story. The premise, about two forms of life on two different colossal titans, is a very creative concept in and of itself. The worldbuilding is also excellent.
You get to explore a deceased titan with organic life growing on it, and that in itself is a cool sentence.
Many likeable and memorable characters, with Shulk and Dunban as standouts.
Loads of side-quests to take that you get payment, experience points, or both for.
Extremely vast environments.
No invisible walls or restrictions, you can literally explore the world as freely as you want in any way that you want.
Fun gameplay, each character has their own benefits and detractors in combat, and there's many different abilities to unlock and level up.
The Monado is a very useful weapon, it comes with it's own abilities and can even show visions of the future so you can prevent one character's impending doom.
You can play as any of the main characters.
Good voice acting in both Japanese and English.
The Definitive Edition on the Switch gave better graphics that more match the aesthetic and extra post-game content.
An affinity system between characters that unlocks extra conversations as it gets higher.
Although the primary enemies, the Mechon, cannot be damaged with normal weapons, the monado can temporarily enhance party members weapons to fix that.
Lots of extra-curricular activities, such as the aforementioned affinity conversations, extra boss fights, and challenges.
The affinity system, while it is a good addition, requires A TON of grinding in order to unlock some conversations, which could prove especially laborious when you've gotten a new character and have to do a lot of fighting with a specific member in your party for hours in order to increase their affinity with another specific character.. Luckily, the affinity system isn't required.
The 3DS version is exclusive to the New Nintendo 3DS/2DS because it requires the extra controls and power that the original models lack.