Why some novels others consider great - I consider to be duds

I've posted a few times recently on FR Leavis's Great Tradition and my own take on the limitations of his viewpoint and on the many types, modes, genres of works of literary fiction that fall outside of Leavis's definition and can be considered "great." My compendious and liberal definition or description of great literature led friend WS to ask, OK, so what isn't great? What books that other consider great literature would you push off the list? This is a much less pleasant question to consider, as my goal - anyone's goal, I guess - in reading literary fiction (for pleasure) isn't to find the duds but to take pleasure in the best works of imagination, not to say no, in thunder, but to join the chorus or, switching metaphors, to unearth the hidden gem, occasionally. But, yes, there are works that others consider great or nearly so (not to mention popular - there are thousands of those) that I couldn't or wouldn't finish - sometimes my limitation, sometimes I think the author's. These duds or phonies suffer from these (and perhaps other) fatal flaws: works in which the characters do not seem believable or credible but rather concoctions of the author's imagination, manipulated by author to say or do things that people would never think, do, or say; works in which the plot seems completely improbable but just designed to show off the author's supposed architectural skills; works that fall apart after the first few chapters or even halfway through, as if the author had a good idea but had no idea what to do w/ his or her idea - or even may have sold a book on an advance chapter and then stopped short; works not only obscure but willfully obscure as to make reading an act of deciphering and suffering; long novels that should have been short stories; novels in which the research is jammed in and showy and not well integrated into the story line; novels that purport to give us a view of another culture or another time and end up doing nothing more than that and that perhaps were sold and politely reviewed in an act of condescension rather than appreciation; works that seem only to show off the author's style or learning with nothing behind that facade; novels usually late in an author's career that seem as if the author is running on fumes and just trotting out themes that served him or her better years ago but that he/she now has nothing new or fresh to say or develop. To name just a few. Do these bring to mind any novels you've read, or not read?