Nier is one of the most inconsistent games I've ever played. It has a handful of good qualities and some design decisions so baffling that it's incredible the game was allowed to proceed to certification.
It's technically a JRPG, but calling that is an awful stretch as it plays out much more like a God of War clone with fetch quests. And courier quests. I guess it has a fishing minigame, which is something that all JRPGs have too, but still.
The story pits you as Nier, out to find a cure for his daughter Yonah's curse/disease while decyphering the disease's link with the Shades, the game's Ico-style enemies. In order to do so you basically follow the bidding of Popola, the town's head librarian, running back and forth from location to location defeating bosses and obtaining pieces of an important item, and so on, as you'd expect from the genre. The twist there is that the relationship between Nier and daughter is actually handled quite well, if sporadically, as is his relationship with Grimoire Weiss, the book he is bound to early on. The other characters Kaine and Emil are similarly well developed, though Emil frequently degenerates into whiny generic JRPG child, helped none by awkward voice acting (also see Karol in Tales of Vesperia).
The pacing of the game is probably it's biggest problem. The opening is disjointed, and the entire first half of the game is remarkably boring as very few story details are revealed. The game seems to know this, too, as subsequent playthroughs begin well past the halfway mark (in terms of overall game time). This has the advantage of letting you skip the early boring stuff, but also makes you wonder why any of it was included in the first place if not for the sake of padding.
Combat is serviceable. You get a regular attack that can be mashed into combos, a dash attack for enemies that block or wear armor, and your choice of weapon (one handed sword, two handed sword, spear), although where there are dozens of weapons in the game there's very little reason to use more than two or three as the stats of the rest are terrible in comparison. But even if you decided to cripple your damage output, the game is not particularly hard. It's made easier still by the magic afforded you by Weiss, which ranges from AoE attacks to bullet hell style magic attacks and an obscenely overpowered lance attack that slows down time while you aim. You can also attach words (basically runes or gems in any other RPG) to your spells and weapons to grant them additional effects, from increased damage to reduced mana use.
Quests are forgettable, whether we're talking main quests or sidequests. Main quests involve a dungeon and a boss - to their credit, the bosses are usually interesting, if not particularly difficult, and a handful of them have battles that span entire areas. Unfortunately, the game world is not very large (ten outdoor 'zones') and every dungeon is recycled at least once - one of them has you revisit it at least three times during the course of the game, where you fight through enemy shades each time and repeat block puzzles as well. Sidequests are almost universally terrible, either requiring you to grind to collect items a la MMORPG or deliver a message or item to another character. Some of them hit some nice character beats, however, and as a reward for completing some inane task you're at least treated to some banter between Nier and Weiss at the end of most quests.
As far as the look and feel of the game goes, it's servicable. Nier is not a good looking game, although technically the graphics aren't awful - it's more of a design thing. Everything is barren, consists of low polygons and indoor and dungeon areas have a ton of recycled elements. As small as the gameworld is, you'd think some care would be put into densely populating it with stuff...anything, really. Instead, there's just nothing there. However, the music - which is absolutely fantastic - almost makes up for this shortfall, and the voice acting and cutscenes are generally well done. Weiss is obviously the game's high point, but Nier and Kaine have their moments too.
There are also some interesting stylistic choices in the game that make it a bit more unique, too. Cutscenes are framed by semi-transparent borders - they don't seem to exist for any reason other than to let you know that it's a cutscene, but it's a bizarre choice. Some of the dungeons fix your camera angle - at times you'll be playing what has essentially become a dual-stick shooter. One dungeon has a fixed 3/4 isometric perspective. Indoor rooms lock you into a side view, and some platforming sections are side-scrolling as well. The game will even throw you into Lost Odyssey style text story sections and text adventures.
It's the combination of all of this that makes Nier work - to an extent. Yes, the combat is kind of boring. Yes, the pacing is all over the place and a good chunk of the game is incredibly boring. Yes, many of the story explanations (when they do arrive) are ridiculous. Yet the way the characters are handled and the overall presentation makes the flaws a bit more forgiveable. No one's going to mistake Nier for a good game, but it's unique enough that it's worth playing if you can look past the flaws.