While I totally empathise with Damien's enthusiasm for the Culture novels, I think his call for a conclusion is as naïve as my expectation that the “Matrix Revolutions” would bring revelation. Of course, it brought only disappointment and an appallingly ridiculous gun toting mechasuits vs “squidies” battle.
The point is that I should never have expected some kind of ultimate enlightenment; a view into or beyond the reality of an computationally superhuman society: post Singularity is by definition unpredictable/unimaginable.
It was, to my mind, made very clear that the Culture (more specifically the Godlike ship/hub minds) deliberately limited themselves from transcending. Any societies that had done so effectively disappeared from the universe. By anthropic principle, the Culture must then be composed entirely of those intelligences who have foregone the next step (perhaps the ultimate change). Many stuck around because they consider it irresponsible to just 'sublime' and leave developing (planetary) societies to fend for themselves. Hence the repeated theme of 'Special Circumstances' policy/practice of interference (in global wars/politics of lesser sentient societies).
For this fundamental reason, there could be no significant evolution of the Culture as a whole; even if great swathes of it's constituent members got bored of their limited life and chose suicide or transcendence, the galactic civilisational niche would be re-filled by something just as Culture-like, because it's identity is defined by hard constraints.
So, viewing the Culture as a series, the only definitive conclusion possible would be galactic armageddon. Something along the lines of the 'Greenfly' that destroy everything at the end of Alastair Reynold's "Revelation Space" trilogy ark. An 'homogenising swarm' as Banks terms it. Alternatively there's the "Iln" in "Matter" (I.M.Banks), a dead/dormant race, with technology on a par with the Culture's, hell bent on interstellar destruction.
While both Banks and Reynolds do a very good job of building an air of terrifying evil around their antagonists (I'm thinking more of the "Wolves" in Revelation Space), the problem is thinking up a plausible explanation for destruction. The Iln remain mysterious, but Reynolds reveals that the Wolves are there to prevent the emergence of space faring sentience during the collision of two galaxies (an event spanning hundreds of millions of years), because it would cause too much fighting and mess (over whose solar systems get saved from annihilation, etc). Not all that plausible to me, but a raison d'etre nonetheless.
Ultimately, the most evil things in the galaxy were intent on the greater good. It seems somewhat impossible to think of a movement of 'bad guys' beyond iterated Nazism (i.e. nutters), which is presumably incompatible with being hyper intelligent enough to be a threat to the Culture.
Besides, the most prominent leitmotif in the (loose) series, for me, is the persistent triumph of the Culture's agents, against any odds. There's always that safety net of benevolent super-intelligence that will win out. Such a strong streak of optimism is exceedingly rare in sci-fi (Banks perhaps compensates by killing many of his main characters, most of the time).
Although the Culture itself is developmentally static, I think it works well as a metaphor for the triumph of (human) intellectual progress: always winning out eventually, like democracy over fascism (WW2). A hymn to the inevitability of a better world, and an explicit vision of a true utopia at the same time. To destroy this in a future novel would be nihilistic or nonsense: a cheap, Hollywood dystopia.
This being said, I am already excited about the possibility of another Culture novel. In fact, given as it's listed on Amazon as "New Culture Novel (Hardcover)" I think I'll pre-order it now, ready for when it's release on my Birthday, February 2011!
P.S. - Side note:
I don't think the Culture novels aren't supposed to be an endorsement of 'Western' military action in developing nations (e.g. the Iraq 'War'); The Culture should definitely not be equated with the USA:
The novels are unrealistic, in that the Culture, as a society, wants for nothing, is entirely self supported and so can have no vested interest in the affairs of less developed civilisations. In reality, countries can always have been said to have made economic, political or security gains from interventionist actions: however evil a toppled dictator may have been, a military action may well not have happened otherwise. A conflict of interests indeed.
In fact, I have been increasingly convinced that externally instigated regime change has historically been for financial reasons: effectively forming a global empire with plausible deniability.
I personally, was pro Iraq intervention: to end it's dictatorship. But I felt that many other instances (where intervention was desirable) were being wrongly ignored, and invasion of Iraq was being justified for entirely the wrong reasons.
In retrospect it seems like regime change is a very tricky thing indeed, almost invariably leaving a wake of violent reprisal; the insurgency and anti-western sentiment being an almost equal and opposite force to the raw military supremacy of our armed forces. Even if the impetus for revolution comes entirely from within, unrest is still most likely. Without the semi-omniscience of Culture Minds, intervention with minimal turmoil seems near impossible. And unlike their “Idiran War”, which they calculated to have definitely been worth while (a number of decades after), it is impossible to know the utility of the turning not taken: whether lack of intervention would have been worst in the long run, or not.
One can only hope that technological change continues to cause ignorance and suffering to evaporate.