The editor/publisher's raison d'etre, at most publishing companies, is to bring revenue into the company. That may sound a little bloodless and corporate, but the way we generate revenue is by signing up promising new writers while also trying to make a buck on writers whose work has a proven history of selling. Bankrupt publishers don't do the world any good, and I've never apologized for being a businessman as well as an editor.
But things get a little tricky in the editor's head. On the one hand, we are looking for manuscripts that, in our opinion, will be purchased by a reading public with literally hundreds of thousands of other options. On the other hand, though, there's a part of us that sees ourselves as opinion leaders. This is where we get into that slippery phrase, "love a manuscript." The sentence "I didn't love this manuscript as much as I need to" usually means "This manuscript may or may not sell when published, but overall it wasn't to my taste, so I'm going to pass." This probably comes as no great epiphany to writers who've heard it time and time again from agents, editors, publishers.
Still, many of us make our decisions based on what we perceive as commercial acceptability. So, over the years, I have been compiling a list (in my head) of the things readers hate. If a manuscript does any of these things, it becomes less and less likely that it will be accepted for publication any a major house (or at an independent house that is market-centric). I've culled these ideas from reviews, from librarians, and even from some reader-response forums at conventions. In many of these cases, "what readers hate" is diametrically opposed to what I personally enjoy in a manuscript. This is where professional judgment has to take over; and usually Readers win.
1. An ambiguous ending. Lord, do readers hate an unresolved ending. Even when a new series is getting started, each book has to be a complete whole. Minor threads can be left unresolved (to be picked up in future books), but most readers can't stand being left at the edge of a cliff. They get even more frustrated when they feel that the author or publisher is sending them the message, "If you want to find out what happens, you'll have to buy the next book." (See point #5 below.)
What this means, ultimately, is that writers have to separate character from plot in series books. The immediate plot has to be resolved, while uber-arcs can continue from book to book. Most devoted readers of series read to follow the lives of the characters, not to be left hanging at the end of each installment.
2. An unhappy ending. Here's one way in which books and movies are similar. Readers hate an unhappy ending - where the protagonist dies, the guy doesn't get the girl, where one is left with a bleak feeling upon closing the book. To paraphrase Oscar Wilde, "The good end happily, the bad end unhappily. That is what fiction means." People who read to escape want a boost, not a downer, upon finishing a book.
I think in most books we know, just know, that the guy and the gal will end up together. What keeps us reading is the possibility that they might not pull it together, even as we know that, because we're reading popular fiction, they will.
3. Too much detail. I can't tell you how many times I've heard readers say, "I can't stand pages and pages of description. And all those historical details - endless details - I just skip over it." I think many readers perceive too much detail as self-indulgence on the writer's part. Are you listening, Jean Auel?
Earlier in my career, I had the great fortune of editing one of the great ladies of historical fiction. (Absolutely loved her, except she always fought me on breaking up huge chunks of narrative into chapters. Didn't matter, though - her public adored her.) What made her so successful, I think, was that she got the level of detail just right. The political element of the time; the way people lived in their daily lives; the architecture of the period - She drew it all with such economy that readers felt they were living it rather than being educated about it. Occasionally during a nostalgic moment I'll look up reviews of her on Amazon and elsewhere... she is still gaining new fans, and her work still resonates. She taught me early on about what makes fiction good and readable - and I have taken these lessons into other fiction genres.
4. Stupidity. Readers lose patience when books become too "unrealistic." The whole reality thing, and how far people will suspend disbelief, could be the subject of an entire book, so I won't get too deeply into the weeds on it here. But there's one area where readers simply can't stand too much reality, and that has to do with the intelligence of the main characters. Despite the fact that we're surrounded by stupidity in the real world, readers simply can't accept stupid main characters, or main characters acting in stupid ways. I think most editors are aware of this, which is why so few stupid characters make it into print for the first time. However, once a series is established, it seems that characters can get as stupid as they like.
Many moons ago I found a terrific manuscript. Its lead was a beautiful, hard-working - but alas, not very intelligent - woman. The way the story unfolded was based on other characters' taking advantage of her lack of intelligence. It was superbly done, but when I took it to the editorial board, I got shot down. Oh, how eloquent I was on my soapbox, arguing "But the writer made this character unintelligent intentionally - it's what drives the plot. It's not a case of writerly incompetence, having a character do something stupid because s/he's too lazy to think the plot through." The editorial director looked me straight in the eye and said "Agatho, Readers can't tell the difference." Lesson learned.
5. Financial Manipulation by Author or Publisher. The most horrific example of this phenomenon, in recent years, was James Patterson going on national TV to say, "If you don't read this book, I'll kill Alex Cross" (or words to that effect). My first reaction was - Go ahead and kill him; see if I care. I thought, "That's it - Patterson just killed his own career." Of course that hasn't happened (I'm not always right, sadly), but the backlash was pretty intense. (As most people know, Patterson built a career in public relations, so he knows more about consumer behavior than I do. I wouldn't even think about trying to manipulate my market that way, but he had no such qualms.) My daughter, God love her, is a big fan of the Twilight series, but she got really annoyed by the fact that Hollywood decided to make two films out of the last book in the series, just to milk some bucks out of the American public. This is why I think all those writers hawking their own work on DorothyL and other public forums probably experience an unseen backlash; folks have become pretty savvy about the way the Internet can be "gamed" (see recent NYT article about phony reviews) and have become skeptical/leery.