Though it might contain some annoying backtrack repitition, it is still infectious, immersive, and charming.
With that out of the way, I absolutely loved this game. The mood, first and foremost, is characterized by the absolutely beautiful musical score that is at once unique and also eerily familiar and reminiscent. No matter how mundane the task at hand could be, it is always made tolerable by the soundtrack underlying the level. Every town has it's own tune and they are all a delight to listen to.
As far as side quests go, there are a lot of them. The majority of them are fetch quests and involve being asked to run to one town, retrieve a specific item, bring that item back to the original town where the quest giver makes something from that item and asks you to bring it back to the town you just came from. However, these never became tiresome to me as it was a great way to make money but was never required, and never felt like I had to complete the side quests without losing footing in the main campaign. The side quests are light hearted and carry their own degree of charm and attractiveness which lets you see a more tender side of Nier if you so choose, otherwise you can just push through the main storyline which is talked about more in the next paragraph.
The gameplay of Nier is what drew me in, but it was the dark and mature story that kept me coming back. The game plays like a classic JRPG meets action title and I have way expected the storyline to follow a cliched line of progression (regardless of what story previews had told me). This is not the case, luckily. The story line is dark and twisting, and the characters, while sometimes over the top, are for the most case like-able. The most like-able of these is the floating book, Grimoire Noir, that accompanies your character and provides insight and comic relief along the way. The book's voice acting never felt like it was overdone and I was always excited to hear what he had to say next, even if it was a repeated phrase. He is also quick to point out the flawed cliches of both the game itself and other JRPG titles, which just proves that Nier is trying to take itself too seriously and is, instead, just around to provide a fun, deep, and different escape into an unfamiliar RPG world.
In the end, if you like JRPGs with side quests, battles that make you feel like a badass, ridiculously powerful and fun magic attacks, tons of weapons and weapon customization, and an emotional, evolving story, then you just may like... no... love, Nier.