I don’t know if I could define speculative fiction if you asked me to. I’ll leave that to the critics. What I can recognize is good fiction, no matter what you call it. Neal Stephenson is one of my favorite authors, so its not surprising that I liked Anathem, but its a very different book than what you would expect from the author of Snow Crash.
Take a first contact story and wrap it in a philosophical discussion and you have the basic idea, though Stephenson is too gifted a story teller for that description to do the novel justice.