Books can be looked at from many points of view. My one most strenous belief is that when I talk about books, you should not hear the plot in too great a detail (unless the author intends the ending to be known before the book begins). Hence, I rarely comment on plot devices or any story development as I feel I could be shortchanging the reader of the review.
So, I have to look (and like looking in any case) at the other aspects of the book instead. One of these is the writing — very important indeed for me, and I hope for most people, but I need to feel comfortable reading the book. It should not feel forced (again, unless that is the intent), and it should feel good. I should get the sense that the author enjoyed writing it, and if their words are put to paper with such skill that I lose myself in their world, even better.
Part of the above is how well the characters come out. I think I here quite often contradict what some other people say, or at least when I have compared reviews on Goodreads it is quite obvious that characters I liked very much were considered incomplete by others and vice versa. I cannot quite explain it, except perhaps I look for the establishment of the character in something more than the written person. They must feel consistent throughout, and they must feel logical. They must have culture (if that is their background), and they must act as if they belong to wherever they are from.
The above is not always the case. However, I have also noted a lot of people have preference with respect to how much text is descriptive vs dialogue (say Tolkien vs Asimov for an easy one here). I think both of these can be similarly splendid, but they must be appreciated for what they are and how they are. The being of Asimov’s characters will come through their words, while Tolkien’s characters get constructed perhaps even before they say their first sentence. That is the difference between various authors.
Next, I am always partial to an interesting story which is interlaced with enough background for it to feel real. This includes an aspect which is not directly related with either of the above, but which is indirectly connected to both of them: there should be some wisdom in the book. No matter what form or method it takes to come across, either the narrator or a character, whether or not it is picked up on and used or not.
But, lastly, and most importantly, the writing must be good enough for me to feel what is going on. This I’d term as emotion. And, indeed, in my reviews I often go with whether something feels right or wrong and what other emotions the story created. How the characters felt and whether they were right is another aspect. This, for me, that a book feels a certain way, is maybe my own classification, but very useful in the sense that if I am feeling a certain way it is quite nice to pick up a book which complements it. There is, after all, no point in reading a fact-oriented history (as opposed to a story-oriented) when looking for amusement or philosophy, or looking for sarcasm from high fantasy.
The above is not perhaps the most perfect description I could give, but I know that my book recommendations follow this. I know my friends well enough and try to recommend books to them if what I felt in that book seems to match what that friend is like. Sometimes I am wrong, sometimes not. Nonetheless, I have always taken great care if I recommend something (or, at least, I would like to think so).
And, I hope, that derives from me considering more than just the plot. For, indeed, the plot is just the first glazing on the house that is the book, and that is also why I nearly always recommend picking an old book up after years have passed to read it anew.