I am not going to summarize this book as I really don't want to provide any spoilers for the third and final novel in the Hunger Games trilogy. Suzanne Collins is yet another author who has lured adults into reading YA books and with the coming movie based on the novels, I'm sure more readers over 18 will discover Katniss Everdeen. I was not in the least disappointed by "Mockingjay." However, I can understand why some reviewers/readers may not fancy the novel. It is dark. It is gritty. It is honest. It does not have any canned 'and they all lived happily ever after' bits in it.
Somber. Sobering. Sad. Heartfelt. Mature. It does have all of those themes/emotions and more. I'm not sure exactly what some readers expected. The book delivered pretty much what I guessed would happen and in the manner it would happen. In my mind, the final plot points could not unroll in any other way. In many ways it was predictable, but not in a cliched or poorly written way. Revolution and war have universal elements in them as well as universal human reactions to the death and destruction they hold.
I was only a bit disappointed with the treatment of Gale. I would have liked a different resolution for him. But, this is Katniss' story, not his.
All in all, I don't think you can measure this book alone. It is part of a series and the three books need to be taken as a whole. Good. Stuff.