I know what you're thinking, but surprisingly it's not "Your book is shit". That at least generates some debate.
"And why did you think it was shit?"
"Your plot line was as transparent as glass, your characters as believable as the Yeti and your grammar as bad as gout."
"Hmm, you may have a point there. Or three."
In my experience, every author will tell you what offends them the most is when someone comes up to them and says, "Well, I'd like to write a book but I just don't have the time."
What, and we do?
By saying this the person is in some way inferring that their time is somehow more precious than the writer's, when really what they are meaning is that they can't be arsed to sit down and dedicate thousands of hours to writing. There's much more exciting things they'd rather be doing.
The depressing fact is that just 10% of all authors make enough money to allow them to dedicate their entire working life to writing. This means that the vast majority of us have two, sometimes even three, jobs (including the writing) to keep ourselves alive. It means we must find or make the time to write outside of work.
For me I write for one hour before I go to work, each morning, every morning. I write almost every evening, at least 8pm until 10pm. I don't watch TV. I don't sit down and relax. I write every weekend, always up at 8am, at the latest regardless of what I had been up to the night before, and write for as long as I am able to during these 'precious family hours'.
Don't get me wrong. I am not complaining. I write because I love to write and I write because I feel this need to. I've learned to be selfish, as all serious writers need to be, and put myself and my writing first. But I don't find I have the time to write because I've got a direct line to CERN and have got them to bend time for me. I have no more time than anyone else. I make sacrifices, as do my family, so that I can write and keep the momentum of my writing up.
The problem is people aren't buying as many books as they used to, so writers aren't getting the sales to earn them the money to allow them to write full time. Therefore they need to write whenever they are able, not when they would like to. But I promise you you'll never hear a writer complain about having to find the time to write, at least not one who believes in their work and why they are writing.
So the next time to engage in conversation with a writer, don't say that you don't have enough time to write your masterpiece. Tell them you've not got round to it yet, you don't know how to start the novel, or that you just can't be arsed. You'll win the author's favour and perhaps generate even more debate than by saying that their novel is 'shit'. Although it's a start, I suppose.