What appeals to me about science fiction are the worlds and societies that authors can imagine; the kinds of future they are able to conjure for humanity; the issues and problems these new and strange times and worlds can give rise to. Themes that are bigger than specific characters and their stories and relationships. And indeed for me it is a shame when a science fiction book puts too much emphases on the latter and leaves the bigger picture unpainted - it then kind of misses the point of science fiction. And of course it takes a learned and scientific mind in addition to the ability to write in order to produce good proper science fiction - something which is coherent and believable and makes you think in new and exciting ways.
A book that I recently and quite by accident came across - Greg Egan's Schild's Ladder - comes pretty close to being an excellent sci-fi book of the proper kind. Set some 20 thousand years in the future, it focuses around the problem of dealing with a viciously expanding new kind of a vacuum, accidentally created by an experiment, that is destroying our universe with its laws and principles in its path. Even though this is the central axis of the book, many interesting details are revolving around it. Space travel through your identity being transmitted as a signal from one point to the other; human and non-human bodies, which this identity can inhabit and swap; completely bodyless existence; slowing down and speeding up of perceived time to alter the experience of events; and so on. It really is a treat to the mind, regardless of some of its weaker sides and despite the fact that at some points it goes onto territory which is quite incomprehensible and unimaginable.