Where do I start? The humans are essentially pets, kept around by the Minds mostly for nostalgic reasons and because it's a trivial effort for them. People love their pets, but don't allow their pets to make life-altering decisions.
All of the main characters are Minds or drones. None of the biologics ever get to influence events, they're just along for the ride.
The humans are controlled, both actively and passively. The Minds don't lower themselves to anything as crude as "carrots or sticks". Oh no, they prefer to use a far more subtle forms of "guidance", such as inventing an artificial language (Marain) that influences the way people think. This is the Sapir–Whorf hypothesis: https://en.wikipedia.org/wiki/Linguistic_relativity
The Culture's main opponents, peaceful or not, are all primarily-biologics that reject machine minds and associated technologies such as field manipulation. After reading the series, you start to get the inkling that perhaps they are right, and that the Culture made a mistake in allowing themselves to be ruled by computers.
The undercurrent, the persistent theme, in all of Banks' works is betrayal, often of the worst possible sort, the kind that leaves you breathless when fully unveiled in the final chapter. The Culture series has some sort of betrayal in just about every book. Often it is the machines betraying the humans in some way. The drone Skaffen-Amtiskaw in Use of Weapons, for example, was deceiving everyone to some degree, even the protagonist Zakalwe. Some of this deception feels unnecessary, taken to the n-th degree by the drone just because it can.
Similarly, the ship Grey Area is notable for being a "meat fucker" in that it abuses (or merely uses?) its godlike power to torture organics it judges as worthy of punishment. This is called out as unusual, but then in A Look to Windward the hub Mind casually strips Quilan's very soul out of his meat body so that he can experience death at the elevated level that only a Mind can. A worse horror is difficult to imagine.
Etc, etc...