As with most sports animes, the plot isn't particularly unusual or exciting - the genre is limited to a pretty standard formula of introduction, practice matches, training, a tournament arc, and repeating the process. The characters are also largely drawn from the typical mould - a cool, highly talented loner, a solid and experienced captain, a kind senpai with wisdom to balance out his lack of talent, an enthusiastic and slightly delinquent-ish enforcer - but they manage to be fleshed out and distinguished beyond their stereotypes. The protagonist, Shoyo Hinata, is arguably the least interesting character, being that there's nothing beneath his earnest and naive volleyball-loving surface, but the rest of the cast makes up for him in spades.
Kageyama ("King") is highly talented and aloof, and in another anime this would be the end of our insight into him, but Haikyuu gives him the familiar neuroses of a high-strung perfectionist who couldn't accomodate his team and wound up alone, and also gives him a satisfying arc by showing him learning to step outside his shell and grow as a person. Another conventional feature of sports anime - a player's position being in some way a metaphor for their personality - is used throughout Haikyuu in a far more interesting way than we'd usually get. As the aggressive and antisocial type, Kageyama would usually be a spiker, but as the setter, he needs to be able to provide service to his teammates in a way that brings out everyone's full potential - to do his job well, he needs to actually communicate and get to know them.
The most magnetic of the side characters in this season, Oikawa, is a former senpai of Kageyama's, and a thoroughly unpleasant, smug, cocky, and psychologically insightful guy, who uses his ability to read people to vicious effect in games. But despite all those seemingly-villainous attributes, he too is a setter, and in his masterful ability to connect with his team, he demonstrates a maturity which makes him more complicated than you might assume. That ability to provide a surprising dimension to seemingly-flat characters is present throughout Haikyuu, which finds a way to make everyone feel a little real.
Tanaka might be a bit of a delinquent, but he's also comically elated by being called senpai, and his superficial (but hilarious) idiocy hides mature resolve and a surprising amount of wisdom. Karasuno's team advisor Takeda might be a volleyball amateur, but he shows both wisdom and adorable ingenuity and perseverance in making sure they get the best practice matches. Unlike the drama resulting misunderstandings and standoffish idiocy in a lot of other media aimed at Haikyuu's demographic, the characters in Haikyuu talk to each other and communicate constructively to reach a shared understanding and help the team.
Aside from its characters, Haikyuu's other main strength is its keen sense of development - where you might initially think "Why doesn't he just spike it like THIS?" of Shoyo's early stumbles in matches, later training and innovation on his part (and others) show an evolution in techniques and strategy which makes volleyball seem genuinely interesting - and I say that as someone with no interest in it. The key to a good narrative is following premises to their logical conclusions, and when it comes to character development, Haikyuu demonstrates growth and change on the part on almost everyone in response to their changing circumstances.
Rather than flaws, what keeps Haikyuu from being a masterpiece are its limitations - the limitations of conventional shonen sports anime and a focus on the sport itself which doesn't allow for the concerns of real life to intrude on it. One of the things that struck me as surprising near the end of the season was the third-years conferring with a guidance counsellor about whether they should keep going with club activities or focus on their exam - this shouldn't have been a surprise at all, as it makes perfect sense, but Haikyuu is so laser-focused on the sport that you gain no understanding of what lives Shoyo, Kageyama, Daichi, Tanaka, Oikawa, or the other characters lead when away from the court and the gym. The fact that it's able to make them engaging (for the most part) despite this is impressive, but given the grounded realism of Haikyuu - its lack of overly flashy techniques and the relatively subdued nature of its characters, who all (besides maybe Shoyo) seem like people you might've known in school - some details of the world outside volleyball would have helped make it even more real and immersive.
Regardless, as it stands, Haikyuu is a well-put-together anime with a strong grasp of traditional narrative techniques and solid characterisation and development, as well as excellent animation and an attractive art style. It's a good advertisement for the value of the medium, and that's more than enough.
