if you want a real answer, basically it's just got charming characters and an interesting story lined with a unique tone to the game overall. there are pretty unique uses of the medium in terms of narrative function that help to drive the themes of the game through extremely well, especially if one sticks through to the completion of the game itself ("100%ing", effectively)
alongside that it's got one of the best soundtracks made for the medium ever so that helps a lot
in terms of gameplay it has a lot of interesting ideas that are never executed or fleshed out as nicely as they should be but hold enough entertainment value in how interesting they are.
they're interesting enough to where a developer like platinum games getting their hands on it could lead to some well-developed, unique and satisfying gameplay over simply being amused by competent base executions of the ideas
to summarize: the hype is built up from the key members of the 'good' parts of nier combining with one of the most lauded development studios in the business- there's a real hope for a game that has that rare combination of compelling and truly unique gameplay, story and concepts with this absolutely bizarre and insane team
also a lot of it is because literally 0 people expected it to happen because the director's games (including nier) have a long history of selling generally poorly- they always have a clear, interesting vision to them that never is executed well and this may be the first (and last) chance for folks to experience the possibility of the vision realized