The basic plot of a novel consists of a main character trying to accomplish some genre-appropriate task (save the kingdom, solve the crime, stop the bomb, overcome obstacles to love, whatever). It follows that one basic way in which a novel can be bad is that its main character can be passive in its main plot, just experiencing things that happen to him or her.
This makes for a bad novel for two reasons. First, it makes the main character unsympathetic (we'll talk more about sympathy another time, but, to foreshadow a bit, we sympathize more with characters who are active). We don't care much about characters who don't care enough to do something. Second, it makes the plot uninteresting, just a series of things-that-the-author-makes-happen-to-some-schlemiel.
Are there exceptions? Maybe; we could argue. But, in terms of film, if you think Forrest Gump or Being There were great movies (I have not read either underlying book), let me ask you this: were they great because of the plot? Or were they great in spite of a weak plot?