Human society changes quite a bit more in The Dark Forest than it did in the first book. Reaction to the knowledge that aliens exist and are on the way causes all kinds of changes: a major shift in economic and industrial output to focus on building up space capabilities, the formation of different political entities, and eventually environmental collapse leading to a massive reduction in population.
There are plenty of advanced science aspects in the story, from space elevators to suspended animation to long-range cosmic observations. Those things are mostly secondary, though, to the main focus on the psychology of the various actors. Characters struggle with despair, mistrust, intrigues, and the fatigue of working toward goals that won't be achieved for centuries. The advancements in technology happen almost in the background by comparison.
I didn't have quite as much trouble with suspension of disbelief in The Dark Forest as I did with the first book. I don't think this is because the plot and character actions are significantly more realistic, but once you've accepted the premise laid out in The Three Body Problem, it's a lot easier to extend it to the things happening in the second novel. The way that Luo Ji finally confronts the alien menace seems pretty flimsy when you look at it closely, but there's been enough build-up by that point that it's fairly easy to accept.
By the end, The Dark Forest resolves most everything that was left unfinished from The Three Body Problem. In the process, though, it creates more questions for the third book to take up.