For me, what makes sci-fi worthwhile is the following:

1. Asking big questions about the human condition, and humanity's place in the Universe.

2. Exploring the frontiers of science.

3. Blurring the line between the fantastical and real by building a plausible bridge between the fictional world and the reader's world.

The Three-Body Problem excels at all of these. Not only is Liu highly imaginative in constructing his universe, but he also manages to firmly ground it in well-drawn and memorable characters, real science, and most exceptionally for me, Chinese history and culture. The book begins in the throes of the Chinese cultural revolution, and this turbulence ripples through the rest of the book. This gave his story, already an  outstanding example of first contact, a unique flavor, and it makes me want to learn more about something I know little about, a sign of an excellent book.

Another sign is that I couldn't put it down. This book really got in my head. I look forward to the sequels!