I hate it when I forget to write a review and then by the time I get around to it I've forgotten all but the most basic details of the plot. That happened a number of times when I was sick and now I HAVE to go back and re-read them. I want to say "I read it, it was okay but not great, I don't remember anything more than that" and leave it at that but my brain just won't let me. One side of my brain beats at the other demanding that I do justice to myself by writing a proper review. Unfortunately I just can't let it go (or one side of my brain can't).
If I get too fuzzy, I'm usually good with a very quick skim of the book to refresh my memory; sometimes though, my inability to remember any of the book is an indication of the quality of the writing and I need to keep that in mind. (It wasn't the case here, btw - the book was a solid read - I just forgot what I had planned to say in my review.)