The issue is one of payoffs for the reader, what will keep them reading. Can a clumsily written book with great characters do it? Can a book with weak characters but wonderful prose do it? And I think the best an author can hope for is that more readers are engaged by his/her prose than not. And so, below, is what I said.
This is something I've wondered about as both a reader and a writer. I consider myself an easy reader. I'm willing to put up with a lot, more than most people I know or encounter online, apparently. I have a basic curiosity that needs to be satisfied. A book can be poorly written or annoying and I'll still want to read so my curiosity re: how the author will resolve the plot will be satisfied. If the book is that annoying, however, I move into skim mode, but I never jump to the end.
My mother was a read to the bitter end kind of reader. I remember asking her how a book was that she was reading. She didn't like it. I asked her why not stop. She said it might get better. We kept up that conversation until she was 5 pages from the end and in her opinion, it never did get better, but she finished it.
And I was just like that. It's only been in the last 10 years, since I got more serious about writing and have less time for reading, that I've been able to stop reading books that are simply not engaging me on any level, and even so, since I'm fussy about what I will read, I haven't abandoned more than 6 or so books over the past decade.
Can I say I'll keep reading for character or plot or style of the prose? Sure, but at the heart is that curiosity thing. A book has to fail to engage my curiosity about it and that's a very subjective thing, for me to not keep reading. But the more a book engages me on other levels, the faster I'll read it. I'm halfway through Michael Marshall (Smith)'s Blood of Angels and he is one author who engages me on every level, via his fully realized characters, writing style, plots, plotting, tension, and, well, he has a way of reaching out of the book and gripping me by the throat and not letting go til I'm staring at the last page, slowly realizing I've finished. I stink at analyzing fiction, but I do see how he's building the tension and while that's something I could do, I could never do it the way he does with his dense, rich prose.
Whereas, an SF author I knew from when she wrote fan fic, has a way of writing books that are so wordy and have such stilted prose that I find myself skimming her books now. I haven't been able to get into her standalones, but I love her series because I love the premise and the protag, and while I think the first two books were the best, I've kept reading/skimming them because I care about that protag and want to know what else happens to him. And mostly, the plot, if not the actual writing of it, provides that payoff for me, if that makes any sense.
In answer to the point that if one area sags, a writer can keep a reader interested by providing payoffs in other areas, I said:
I agree, but it will vary from reader to reader. Some just want or expect more than other readers. Some will give up when one area sags, because they've been thrown out of the book. Others will forgive and continue. And still others might not even notice.