TL;DR: If you have even the slightest interest in either MetroidVania games, Platformers or anything cutesy: This is a must buy for you!
If neither of those apply to you, give it a try anyway, you might still fall in love with it as I did :)

Pros and Cons:
+ Stunning visuals and great animation quality
+ Amazing soundtrack
+ Emotional story
+ Relatable characters, despite the fact that barely a word is spoken
+ The design, "voice" and animations of Ori will make even the most heartless bastard want to squeeze and cuddle her / him
+ Hard as balls, but fair (for the most part)
+ Some unique mechanics (for example guiding enemy projectiles while using them to change direction / boost your jump midair)

- When you beat the story, you can no longer go back to your save.
So don't put the last bit of exploration off until after the last story bit, or you'll have to play through the entire game again for that one single secret and the 2% of map space you missed ><
(yes, I'm salty...)

Story:
I usually talk a lot about the story in my reviews, but for this one, I kind of don't want to tell too much.
It's not a huge story and it's perhaps not quite as "perfect" as some people say it is, but it is very emotional and deserves to be experienced unspoiled.

Gameplay:
At its core, Ori is a MetroidVania game, but with a heavy emphasis on precision platforming.
The controls are tight and precise and traversing the environment feels really fun.
The difficulty is hard enough to really give you that sense of accomplishment that many gamers crave, without being frustrating.
It can get really intense at some points though.
You set your own checkpoints, which is interesting and helpful, but can be a bit disheartening when you die and realize that you forgot to set one for half an hour ^^"

You start off with just a jump and a basic attack, but as you go through the game you'll get a lot of different abilities, some of them very unique to the genre.
One for example allows you to bounce yourself off an enemy projectile, in any direction while sending it the opposite direction.
You also have an upgrade system where you can spend points to enhance your abilities and gain new ones.

There's a ton of secrets to find, like health and energy upgrades, or more points to upgrade.
They are often not easy to get to and there's a real satisfaction to finally reaching some of them.

Of course, there will be a lot of backtracking if you're planning to get all of them, as is typical for the genre.
But it doesn't really feel that way, since after gaining new abilities, the way you traverse the previously done sectors will have changed completely.

Level Design:
The level design is close to perfection.
There's a real flow to traversing the environments if you do it well and there's a lot of secrets and hidden passages to discover.

The main path usually leaves some room for errors, so it won't be frustrating, while the secrets and shortcuts often require perfect timing and control to reach, giving the people that want it the ultimate difficulty, without putting up roadblocks for lesser skilled players.

Many of the secrets that require abilities that you'll learn later on are kept just slightly out of reach, giving you that desire to come back later when you can finally get them.

Despite how perfectly planned out all the sections are, it still feels very natural, like that's just how this world is, which does a lot to help the atmosphere.

Soundtrack:
The soundtrack is a beautiful mix of orchestral music and choral vocals that carries a lot of emotion.
Whether calm, cheery, haunting or intense, it always fits the current situation perfectly.
It's rare for me to buy game soundtracks, but this one I bought and loaded onto my Mp3 player, so I can listen to it wherever I go.

Visuals:
The aesthetic in this game is wonderful. Honestly, it's perhaps the most beautiful looking 2D game I've ever played in my entire life.
The environments are varied and interesting, the characters are adorable, the lighting is intense and colorful, the background paintings are stunning and the animations just make everything feel a lot more alive.

Final Thoughts:
I played through this game in just 2 sittings and I can't wait to play through it again.
Even tough I usually dislike MetroidVania game and I'm not too fond of platformers, this has easily become one of my top 10 favorite games of all time.
Despite the fact that I usually don't give a damn about achievements, I want them all.
Despite the fact that I have no interest in Steam playing cards, I will collect them, because I want the badges and backgrounds.
If you didn't buy it because it didn't seem like your thing, give it a try anyway.
I think you won't regret it :)