For me social significance is that the book has something to say to human society in general.
I'm going to respectfully disagree. While all literature reflects on the human condition because it is written by people for people, SF is absolutely unique in being able to pose an utterly unprecedented predicament and then explore it. Whether it is
Blade Runner examining the morality of artificial intelligence or
Dune exploring what happens if an actual ubermensh is created, SF isn't constrained to the human condition of today or yesterday. It doesn't have to be about people at all, but what happens to aliens or machines. It can be about the downsides to incredible inventions like Vernor Vinge's bobbles or what a real utopia looks like per Iain Banks. I think it would be terrible if the best SF story has to meet the criteria of some other sort of fiction in the "what's it about?" category. It should be about something so unlikely that it may never come to pass, but so fascinating because the author writes about it like it already has.
To me, the best SF novel stands on its own, both as a single work of fiction and without reference to its pedigree. I don't care if it is the first spaceship story - I would rather read the best spaceship story.
>>The best SF should be unique. Whether it is the world, the plot device or simply the viewpoint; I want to feel like I am in new territory.
>>It should be well written. That doesn't mean only one thing, but it may mean that what you're reading stands out line by line. That you come away from reading even a short passage and feel you have still communed with the heart of the book.
>>It should thrill. Not just be "pretty good", but engrossing, page turning and maybe even devastating in its climax. You should care, even though the events you just read about couldn't be more alien to your 21st century life.
>>It should speculate. Not just suggest a fantastic device, but see that through to all the different ways that thing the book pivots around can warp and adapt to create even more twists and turns in the plot. In
A Fire Upon The Deep, Vinge doesn't just imagine a kind of auditory group intelligence, he spins out many different possibilities for how these beings could live and then change with technology. The best novel should have a good dose of this kind of clever extrapolation.
The best SF novel should leave you giddy, considering re-reading it immediately and definitely revisiting it throughout the years. No one novel is going to get universal agreement, but some novels at least live up to the call of fascinating speculation that keeps our attention on every page. That's where we'll find our finalists.