These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Yamaguchi's growth as a player, culminating in scoring four consecutive points with service aces during the third match with Aobajousai.
Ensemble Darkhorse: Sugawara. He ranked fourth in the first popularity poll despite not playing in any matches since Kageyama became the regular setter (he only played in one match for a partial amount of time during their official match with Aobajousai). In their second official poll, he got second, just behind Hinata and beating Kageyama, who got first in the last poll.
Yamaguchi too. Despite being Tsukishima's Tagalong Kid and not having a lot of action, his feature moments and boundless effort for growth seem to really make him stand out because he got seventh place in the second popularity poll, right behind Noya, Oikawa, Tsukishima, and the top 3 mentioned above.
Akaashi is pretty popular as well. He got 14th place when Bokuto, who does a lot more during the training camp arc and is their team captain, got 18th.
Nekoma in general is very popular, but Kenma is probably the one who takes the cake in that team. It helps that he’s somewhat really relatable to many viewers (especially western fans), due to being introverted, socially awkward and fond of gaming.
Fan Nickname: Hinata is often referred to as "Sunshine baby" by fans.
Kageyama is referred to as "King of the Dorks" by fans.
Ho Yay: It's kind of inevitable due to having a primarily male cast, but it's less then what one would expect.
Hinata and Kageyama's Vitriolic Best Buds friendship attracts a certain type of shipper like flies to water, and it doesn't help how they quickly grow into an extremely-close partnership that others confuse as lasting much longer than it has.
By the way they're introduced, Nishinoya and Asahi acted as if they were a couple who are trying to make up after having a big fight.
There is some between Kuroo and Kenma who are childhood friends despite being a year apart. Kuroo trusts Kenma a lot with his skills so he never doubts him.
It’s almost inevitable to see some of this between Sugawara and Daichi, thanks to their status as Team Mom and Team Dad of the team and Sugawara being Daichi’s Number Two.
Furudate's habit of introducing characters in duos provides fans with a lot of conveniently non-intersecting, ready-to-ship pairs with unique dynamics. In addition to mentioned above, there is Tsukishima/Yamaguchi, Takeda/Ukai and Bokuto/Akaashi. Tanaka, being the only Karasuno regular without an "obvious" pairing sometimes gets shipped with Ennoshita.
It's also worth noting that every setter except Kageyama is commonly shipped with his captain/ace.
Launcher of a Thousand Ships: Hinata, good God Hinata; there's Hinata x Kageyama, Hinata x Tanaka, Hinata x Tsukishima, Hinata x Nishinoya, Hinata x Sugawara, Hinata x Yamaguchi, Hinata x Kenma, Hinata x Kuroo, Hinata x Lev, Hinata x Aone, Hinata x Kiyoko, Hinata x Yachi, Hinata x Oikawa and the list goes on; quite literally, Hinata could get shipped with anyone.
Shipper on Deck: Sugawara seems to ship Kageyama and Hinata; especially when he mentions Hinata's expression when he hits his spikes isn't the same as when Hinata hits Kageyama's spikes and only Kageyama can get Hinata to use his abilities to 100%.