I was thinking more in the line of those who say "It is a crap book, complete rubbish" etc about books that have won awards, for example. Would you not agree that it is likely that these people haven't understood the book, rather than a widely acclaimed book being complete rubbish?
Hmmm.... It's possible, of course.
But I'd be quite pissed off if someone tried telling me that the reason I dislike the writing of, say, Edward St Aubyn or Justin Cartwright is that I haven't understood it. I know that this writing is overrated, that it's of the same quality as much run-of-the-mill chick-lit of the sort that has mild literary pretensions. (You know the sort of stuff I mean: generally produced by glossy mag journalists with a bit of education, a reasonable turn of phrase now and again, but not a great deal to say.) But for some reason it's trumpeted as outstanding writing. Which it just is not.
Generally, if I don't enjoy a book, then I'll accept that:
a) It's the wrong time for me to read it. It doesn't resonante with me at this point, or I don't have the time to put in the hours and energy to 'connect' with it.
(That might sound really wanky. But still...)
Generally I'm happy to take this approach with more challenging literature.
Alternatively:
b) The book is a crock of shite.
e.g 50SOG, etc.
Don't know if you've read The Slap, Cote.
Before I read it I was under the impression that it would be quite an interesting, well written book. Quite a lot of media discussion of it presented it as such. But it's shit. Just shit.
What am I trying to say 
Um...
I think the Cloud Atlas thread is a great idea.
Could be a good idea to start similar threads about any book, I guess, particularly if it's not written to a convention that lots of readers with be familiar with.
But I really baulk at telling people they haven't understood a book.
This is probably going to sound condescending to someone, in some way, but I think if people want to go back and re-assess a book then they should do that in their own time, without being told to.