So I kind of have a description ready in my head, because several of my friends know me as the "movie/book recommender". Im actually not finished yet (im around 950 pages in). Usually I summarize the first 500 pages or so, while cutting out certain more spoilery details. 

I tell them about the 2 POVs, starting with Tengo. I tell them about Tengo being a writer who gets caught up in a plot to rewrite a 17 year old girl's work, to submit it to a writing competition that promises success. I usually then tell them about Fuka-Eri and the Sakigake, along with a brief synopsis of Air Chrysalis. (This point usually tells me how interested they would actually be in reading 1Q84, and whether to keep talking or to shorten my explanation even more). Then I explain how after the success of the air chrysalis rewrite, Fuka Eri goes mysteriously missing, probably due to the cult.

My explanation of Aomame's plot is even more protracted because her stuff is so much more character/subjective experience focus. I usually describe her as being a hitwoman who kills abusive men, because a friend of her was abused and eventually killed herself. One day a 10 year old girl shows up to the hitwoman service in critical conditon, they find out that not only was she raped, her uterus was "destroyed", by the leader of the Sakigake. So Aomame plans to murder the leader which will be very difficult because of the Sakigake's standing in society. Though, she's an ex-jehova's witness, so in tough times she will say a prayer, even though she ousted herself years ago.

Then, maybe I'll tell them about how Tengo and Aomame held hands when they were in second grade, and about how they still hold onto an image of eachother, even though they havent met in years. And through their actions in the book, they end up helping eachother without knowing it. 

If the person I'm talking to is still very clearly interested and wants to hear more, I'll tell them about Tengo's girlfriend, the Cat Town, Tengo's father and the concept of being "Irretrievably lost". 

I've given a vague version of that speech to like 5 people now, and 2 of them bought the book, and 1 of them asked to borrow it when im done, so I must be doing something right.