Right then! Let's pick up where I left off, shall we?
I was busy contending that SF is not necessarily defined by technology, be that steampunk difference engines or ultra-high-tech weapons systems. Some examples are now required, lest I fall into the trap of stating things without evidence. Let us consider the exemplary SF Masterworks series of books, a library that, frankly, no SF fan should be without. In its list of publications, we have works such as Bring The Jubilee by Ward Moore, which features time-travelling Confederates, The Invisible Man (H.G. Wells), and Sheri Tepper's Grass. All of these feature technology as a central part of the framework and plot of the novel, be it the Invisible Man's chemistry set, or Sheri Tepper's colonised world.
Yet they also publish the post-catastrophe novel Earth Abides by George R Stewart. Technology is not a central part of the novel in any way; indeed the protagonist survives the disease that destroys civilisation because he is lying in a cabin up a mountain recovering from a snakebite. There can be no doubt that Earth Abides is SF, but why?
My answer is this: It is a vision of society stripped to the bare essentials, a future history showing us how thin the veneer of civilisation really is; telling us how quickly we would regress in the event of the complex framework of the modern world collapsing. It holds a mirror up to us, and forces us to consider ourselves in a cold light. SF, particularly Analytical SF, is first and foremost about contemporary society and imagining future societies, be they dystopic or utopian, or simply mundane.
Literature may argue its case for exposing the human condition, but I would rather read Earth Abides or We than suffer through On Chesil Beach or force yet more overrated Dickens into my brain. SF, simply put, is this:
Any work that utilises an imaginary society, be it past, present or future, to analyse contemporary society for entertainment or provocation.
It differs from literature in that literature roots itself firmly in the now, or the past. Occasional flirtations with fantasy such as the Magical Realist school of Latin America and Spain notwithstanding, literary fiction is often unable to offer the same depth of analysis as SF because it is stuck within the contemporary society it attempts to discuss. That said, there are many notable exceptions and I do not wish to claim that ONLY SF can provide a coherent critique of society; for every Earth Abides, 1984, or We, there is a Lensman, Stainless Steel Rat or Gaunt's Ghosts - equally, for every godawful upper-middle-class self-obsessed navel-gazer of a novel there is a Cannery Row or a Half of a Yellow Sun.
However, in the end, once you get past the death rays and space empires, SF is all about social analysis, not the technology that provides merely a framework of otherness.