I was promised a masterpiece. A novel full of well crafted dialogue and deep story with a social commentary. Needles to say I got none of that.
The whole time while reading this I was waiting for the breaking point, the part when it gets good, the big twist or something, as I couldn't believe this is what the huge amount of people are praising. Yet here we are.
Oregairu is a harem light novel. No matter how much it tries to hide that with pseudointellectual monologuing, it is still a harem LN. Of course, I'm not saying that anything harem is inherently bad (look no further than Monogatari Series or Sayonara Zetsubou Sensei), but Oregairu certainly failed to make the genre interesting.
In other reviews, you can see "this isn't your typical romcom novel" being noted a lot. That's a lie. This is your typical romcom novel. The only difference there is that the protagonist is (self)described as a loner and loser. A loner and loser yet he gathers supporting characters around and 2+ girls hinting theirs interest in him (this is a romantic comedy after all). You guessed it, it's just a self-insert character at his core. A character for the unpopular folks to identify with and to fantasize how they'll get surrounded by girls too. And Hachiman is self-insert not only for the reader, but also for the author, as the author himself even mentioned that in the afterword. Everyone else is just a background character for the MC to interact with, really. Yes, even the secondary lead consisting of the two girls. And have I mentioned considerable chunk of the whole content of the novel is the MC fawning over a trap? If that is your thing, good for you, but otherwise be beware you might cringe a lot.
I suspect that the reason for mostly positive reception is that almost nobody who didn't like this from the start is going to stick around to actually finish it. And it is the right decision, there is no improvement or reward for forcing yourself through the novel even though you do not like it. The only reason I haven't dropped this is because of my personal no drop policy, and even then I was tempted to just drop this numerous times. It's not outright awful, but it is boring most of the time.
The social commentary, the bellowed philosophical and psychological aspect of this franchise. It just turns out to be a shallow "people are bad and people in a group are even more bad" rambling. The philosophical level of Oregairu is unironically on par with "We live in a society" memes. It brings nothing new to a table for your understanding of society if you made it through a highschool. As mentioned above, all of this really feels just as an excuse for the reader themselves to feel cool - "Those normies don't even understand how cruel society is", "I'm actually totally smart and capable, I just hide it because I'm a lone wolf", "Girls would totally like me if they got to know me more" etc.
As usual with light novels, unless I'm reading an official translation, I'm unsure if the writing style is a fault of the author or of the fantranslation, but I'll have to make do. The writing is bland and there are almost no descriptions, just a string of dialogue and inner monologue of the protagonist - even more than usual with light novels. Contrary to what their recommendations might try to convince you of, the style is in no way similar to Monogatari series. If anything, it's more Hyouka-tier, and that is no compliment either. Popculture references are sprinkled over the novel in attempt to make it more interesting, yet (especially in the first few volumes) the references follow the pattern of "What am I, X from Y?!" or "I see, you know about X as well". This is not a hyperbole. The beauty of references is that you get them only if you know what it references, this is just smashing them in your face. Thanksfully this reference "humor" is used less in later volumes.
The illustrations are amateurish and unimaginative. It's mostly just the characters, well, standing there. As if to complement to writing, the art style is also bland. And just a little pet peeve and again mostly in the first few novels, the boob physics are hideous and on a level of bad ecchi. Though to be fair after finishing the series I can say that the art actually improved, so at least that's something positive.
Overall, there seems to be a lot of readers enjoying this, so you might very well become one of them. Just be warned that there is a chance you might be unpleasantly surprised.