There are many posts and articles out there about how long to give a book before you give up. I personally used to always try to read the whole thing. Then when my friend had a closet-full of Harlequins, I amended that since many of the Harlequins were formulaic (and not in a good way) so several of those I read the first few chapters, skipped ahead a bit, read a little more, and then skipped to the last few chapters. This way I could give the stack back at the end of the sleepover. Occaisonally I liked one of them enough to read without skipping.
At some point I realized that not only is not every book for me, but not all the books I think are for me are going to turn out to be for me. (In other words, sometimes I will bake a bad purchase.) So, I made the hundred page rule. If I wasn't feeling it after one hundred pages, I was giving up. Over time this has morphed into more of a sixty page rule. It is not that I am more impatient. It is that I feel I better understand myself. If it hasn't appealed to me yet, then the chances that it will are extremely slim. And there are plenty of other books out there that I will enjoy without trying. So many, that it is likely I will never be able to read them all. (But I am going to try.)
But there are a few indicators that I have noticed:
Use of italics to indicate flashbacks or thoughts. I recognize sometimes this is a publisher or editor decision, not an author decision. But nonetheless, if font is needed to help me figure out what is going on, I consider it a bad sign.
Use of a ridiculous stereotype. This varies for different people. I personally am tired of the girls don't like sports one. (Yet another reason I adore Nora Roberts). I know that some girls don't like sports. I don't insist that all female (or male) characters like sports. But don't chalk it up to gender.
Use of a child as a plot device. Characters that are children should act in a way that is consistent with their age. They should not appear only when they can ask an eye-opening or uncomfortable question and then disappear or fall silent for the rest of the scene. In other words, if you are going to include children, they should be realistic and three dimensional.
Use of the phrase "over and out". Here's the thing. In radio-speak, over means, "Your turn to talk". Out means, "I'm disconnecting" or hanging up or whatever. So to say over and out is contradictory. Anyone with any training in that would know that. This is something that is very easy to find out. I realize I'm picking on a small thing but it it representative of the whole. If there are glaring inaccuracies, that it appears to me could have been cleared up by some basic research, my patience nears it's end even more quickly.
Use of inappropraite regional references. This is very similar to the above, although it only applies when I have familiarity with the area in which the book takes place. I love Luanne Rice books in part because so many of them take place along the Connecticut shoreline, an area I am very familiar with. As is she, so while she has created a fictitious place in the shoreline, her descriptions of the region have special resonance for me. By contrast I just finished Julie Garwood's Killjoy and the characters use road references that we don't use here. (Overlooking the fact that she doesn't tell us which FBI office they're in until the book is two thirds done.) Again, some research or a local reader could have helped that out. It's not a huge thing, and certainly wasn't my biggest issue with the book, but it pulled me out of the story.
Use of caricatures. If the character isn't fleshed out beyond the broad strokes - oh look, she's crazy, oh look he's gay - I don't want to waste my time.
Use of unnecessary stupidity. Often in suspense there are things that we the reader know, or can figure out, before the main characters. Sometimes that is simply due to us knowing basic rules - someone is after them, it's probably someone we've been introduced to, and so on. But there reaches a point where there have been so many clues, that the only reason for one of the mains to have not figured it out, is because it would screw up the plot. Which means the plot is the problem here.
I'm sure you've come up with your own lists and reason, but those top mine.