EDITED (07-09-2017): removed the minor spolier alert. Edited some comments.

Gameplay:
Though very straight forward, the game mechanics is actually interesting. I'm not a big fan of plantformers because I don't enjoy jumping puzzles and testing my reflexes. Valiant Hearts have some of these elements, but mostly it's at your own pace relaxing puzzle solving. Yet, for those hardcore gamers, this game is also not childplay, but there are over 100 collectibles which should challenge those who strive for perfection.
Please note that there are some minor bugs where you could get stuck. Simply restart from checkpoint and they are nothing gamebreaking.

Graphics:
I believe the sample images should say enough. Please pardon me for my limited knowledge in artistic terminology, but this game has very artistic sytles. Characters are very cartoon-like but backgrounds are simply realistic in its own way. I've been a fan of realistic graphics in games for years but this game's style still amazes me.

Audio:
I would say the audio for this game does even better than its graphics. Every BGM fits so well for the scene and you can really fell the mood while playing. The soundtrack is available on Amazon and I won't urge to buy it because you will do so after finishing the game.

Story:
Let me put it this way: even without the amazing soundtrack, this game's story already guarantees its highest rating from me. It's hard to comment on the story without spoiling anything, but it's a war story that you don't normally see in other games.

Verdict:
The total playthrough is around 6 hours. I suggest you find a day when you don't have any rush to other things and play through this game immersively. Try to pay attention to all the cutscenes and examine any collectible items you come across. I promise you'll fall in love with this game.

100/100 recommend.