In general, you know from the first few pages if a book is going to interest you or not. I know I do, but so often I continue reading a book I have very little hope of enjoying for no other reason than curiosity. I want to know what happens, even when I don't care, and I want to search for morsels of wisdom within the story.
I am obviously delusional.
What makes a book 'bad' is subjective. For example: plenty of people adore books about vampires, but I usually don't. I don't mind them so much if they're one part of a book, but the main feature? No. It doesn't help that most books involving vampires are also romance, a genre I'm not a huge fan of. It's not to say I never, ever read romance, or vampire books, but it's rare.
A lot of people will disagree with this, but I don't think a book has to be well written to be good (although it would probably go from good to great if it was.) That being said, there are different degrees of incorrect language use, which cause confusion on different levels.
A few typos are easily overlooked, but encountering them on every single page is a bit much. Then you get the books which read something like the oddly translated instruction manual for a television. Even though every word is correctly spelled there's something about the arrangement that turns every sentence into an indecipherable tangle of words.
Sometimes I feel like I'm watching the early episodes of a talent show as I wade through the literary equivalent of all those people who think they can sing, but can't. I used to be the nice judge, the one who kindly tells the contestant to practice, take lessons, and return next year, but lately I'm more Simon Cowell. If the first chapter doesn't grab me in some way then I move on to the next book. That TBR pile isn't going to read itself, and reading without enjoyment or learning is pointless.
For the same reasons, there are genres that I'll always be wary of. After all, this shift in thinking was prompted by a book I read a few months ago from a genre I don't normally read. While I don't blame the chick lit genre for one frustrating book, I can simply acknowledge that it's not really my thing. Maybe I'd change my mind if I read a few great chic lit books, but nothing I see in the blurbs ever jumps at me.
When I look into those first few pages of a book, I'm looking for something to fill my book junkie craving. No longer can I justify to myself why I should continue to read something that isn't satisying me. Why waste those hours of my life on an act which should be pleasurable, but feels like an ordeal?