Nier is not an easy game to review at all because its a far better experience than it is a technical showcase, and the areas where it shines are ones that are unusually subjective in terms of appreciation.
I mean I can sit here and tell you that Nier has a great story, characters, and an unforgettable soundtrack but you might not only disagree with my assessment, but consider those things to be something you don't especially care about in a game.
Technically there are a lot of bones to pick at because its trying to do a lot of different stuff gameplay-wise with what clearly isn't a state of the art graphics engine. The viewpoint is constantly changing (from a Zelda-ish third person, to top-down twin-stick shooter, 2.5d side-on platforming, to Res evil style fixed cameras, and more) and some parts simply work better than others; Its a really mixed bag.
What it boils down to is that Nier's jack-of-all-trades approach doesn't allow any single element to shine brighter than its inspiration. Now I think that in this instance that the whole is far greater than the sum of its parts, but I guess a lot of reviewers took the opposite view.
Nier is also a particularly reviewer confounding game due to the way its structured. The most brilliant thing about Nier is unquestionably the perspective shifting replay leading to ending B, something I highly doubt many reviewers are even aware of because it occurs only after story completion.
This sadly is why games are front-loaded these days, because there's no guarantee that people (particularly reviewers) will explore beyond what is immediately offered.
Ironically a few reviews cursed the game's "bad" pacing because they binged on the game's deep stock of side-quests early in the story, presumably out of a misguided sense of thoroughness. No real player should ever do this inadvertently because the game always shows you where to go next to progress the story, and frequently tells you (via Grimoire Weiss snarky comments) that you have better things to do.
Yes, the signposting at times could be better (i.e. the infamous Joystiq non-review over the fishing side-quest) but a missing hint prompt is a minor inconvenience when there's a conspicuous red cross on the mini-map showing you -as it does from the very start- precisely where to go next.