I think perhaps Iain Banks' experience of being an imaginative person underpins these themes. Being imaginative can 'put you into the shoes' of another person, and hence connect you to them. But, you aren't really in their shoes are you? You are running a model of what it would be like to be them, in your head. You are constructing a world of puppets, in your own mind, just when you are trying hardest to connect to other human beings. And interacting with real people is so frustrating and futile compared to controlling the infinite puppets of the imagination. The people in Walking on Glass stood with their heads in imagination, and couldn't tear themselves away to experience the boring real world. The protagonist of The Bridge had to leave his dream and face the fuck-up he'd made of his life.
The theme of torture also runs through the story, and torture is an example of turning another person into a puppet of your imagination. You expunge their identity and impose your own words onto them. One of the main protagonists is a torturer, and interestingly the voice-actor gave him an accent close to Iain Banks' voice, and he has the nickname 'The Philosopher'. The person he tortures is himself. Another person manipulates puppets and it seems that 'upstairs' real people are doing what he makes the puppets do.
The premise of the story - a secret organisation which sends its agents through a subset of the infinite fractal set of possible worlds - gives vast scope. The plot is a power struggle between two senior women for control of the organisation. Our main p.o.v., a male assassin called O, gets caught up in this, being seduced and used by both ladies. Other voices include the torturer and the only character from our world, a hedge fund dealer called 'Ade' who aids the goodies. I liked Ade, though he eventually gets crushed to death by his own possessions. In the Cayman Islands.
The story works because Banks can describe the far flung scenes and contrasting universes so vividly and economically. He goes a wee bit over the top from time to time, but that's what you expect. The political struggles of the day, I mean of today in this world, are given a suitable trot-through in various allegorical masks. Puppets, torture, terrorism. I think he's teetering on the brink of letting plot go altogether and writing a story that works entirely through metaphor and suggestion. Where he doesn't explain, those are the bits that work best.
What are the flaws of the novel? I think when he does try to explain, some of the interlocking story-lines don't make much sense, and I felt it all wound up in rather a peremptory fashion in the last sliver of the book with a Deus Ex Machina which sort of rebooted everything. The good ended happily and the bad, unhappily - that's what fiction means.