This is the second time I've read Diaspora at the beach.
The first time was at least ten years ago. I found it at the beach house, and read it because it was nothing like anything I'd ever read before. The book, at the time, completely captured my attention; it also made me furious.
I thought it was impossible for programs running on a computer to have true intelligence and awareness. Thus, I took the work to be some manner of anti-religious tract. Which it is, to a very small degree.
It's hard to express what captured me about the book. I think a big part of it the depiction of a future that is good: humans survive, thrive, live long and beautiful lives, explore the universe, and so on and so forth, in a rigorously imagined world. I think part of it might be the book's expectations of its readers: if you are not willing to be interested in math, exploration of the fundamental physics of the universe, and in characters who are motivated by the same, then the book just will not interest you.
Perhaps the book is attractive for the same reason HPMOR or Atlas Shrugged or The Chronicles of Narnia are. They all give pictures of how the world is, and how you should act given how the world is, in a way that smaller works do not. There's a unity of narrative and metaphysics which is rarely found; a picture of how the world is, and the patterns of emotional response that let one flourish in such a world.
Reading through it now, there are certainly flaws. The denizens of Diaspora avoid runaway replicator scenarios, as far as I can tell, by finding them... low class? There's a complete indifference to the kind of internal dangers that might take place in such a world, which from my current perspective -- having read a fair amount of Hanson and MIRI literature -- seems grossly ignorant.
Even so, I still love the book. I still don't get all the math in the book. But it remains one I plan to read again.