Stephenie Meyer has described The Host "a science-fiction story for people who don't like science fiction" (source). So what does that mean?
A lot of science fiction is rooted in, well, science. It takes existing scientific concepts and extrapolates them to their inevitable conclusion. Usually, that means bad news for the human race.
But The Host doesn't have exciting new technology that many sci-fi works have, even though it explores a Bad News situation. It also explores themes that only this genre can tackle, like what happens if someone else is living in your body? How would your family react? Your former lover? The situations may not be realistic (at least we hope they're not), but the emotions within it are. This book has heart—we're just not sure how many.