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.
Boring Invincible Hero: A common view of the Culture, in universe and out. Whilst the Culture is easily one of the most powerful meta-civs in the galaxy (possibly others), it is the first to admit that the Sublimed have them down each time. So they don't bother messing with them. And the Culture only stay "invincible" because there are people, drones, and Minds, who are capable of doing anything necessary to protect it's continued existence and interests.
Freud Was Right: Played with in Excession with dROU Frank Exchange of Views.
Ulver Siech: "It looks like a dildo!"
Churt Lyne: "That's appropriate. Armed, it can fuck solar systems."
Mary Suetopia: One thing that makes Banks such a great writer is how he averts this: he is the first to admit that the Culture is the "ideal society" in which he dreams to live, and he is also the first to show how imperfect it is. Specifically the possibility for complete, mind numbing boredom, and the repeated claim (by both citizens and outsiders) that The Culture in fact cannot stand cultures different to its own, and so seeks to "sanitize" the rest of the galaxy. Staving off this boredom and giving purpose to the Culture's existence may be Contact's real function. Also, the utopian nature of the Culture is presented as entirely a function of their post-scarcity economics. When they find anything that actually is scarce, they are only too ready to compete over it. e.g. the Minds scheming and fighting over the titular Excession, or the various lengths Culture citizens go to in pursuit of tickets to the first night performance in Look to Windward. Ultimately, Banks simply acknowledges that even the Culture can't be all things to all people; it's just as close as the author can make it, while still being mindful of the reader.
Tear Jerker: Quite a few of these have taken place throughout the series. One of the most notable would be, for example, the conjoined fate of the Masaq Hub and Quilan in Look to Windward Or the real Zakalwe's backstory in Use of Weapons In Matter there was also the kicker of Oramen's sudden death, along with Ferbin's later self-sacrifice