1) Nothing happens in the opening pages. I don't care how gorgeous your writing is, no one will keep reading about a bunch of characters that are going nowhere. Ditto for pages filled with pretty scenery. Start out with some interesting action or problem, and hook your reader. Readers don't care about what your hero is wearing, that he took a shower, or what he had for breakfast. They will care that he found a letter from his dead father in his mailbox that morning.

2) Too much happens in the opening pages. A beginning with graphic violence rendered in grisly detail is a great way to lose readers. It's too much, too fast. If you must have stomach-turning doses of gore give the reader a chance to start to care about the characters first, and then you can up the tension with gruesome tidbits. Readers like to be scared, not sickened.

Too many characters. If I have to make a list to keep track of who's who, you've got too many characters. The reader will be confused and the story will suffer. Let's say you're writing about seven brothers. Each may have a good friend and a girlfriend or wife. The wives might have friends. There may be parents or co-workers. That's a lot of people to keep straight. (If you have to have that many characters be sure to use names that are strongly different from each other. Never use all the same sounding names or names starting with the same letter; Kark, Kanya and Kumbla.)

3) Info dump. Your story barely gets going when the reader is saddled with pages and pages of back story. This bogs down the pace and is tedious to read. Once we get engrossed in the story, a little bit of back story will add a deeper understanding of why your character is acting the way he/she is. The key words above are 'a little bit'.

4) Multiple viewpoints and multiple plots. Too much going on too quickly will bewilder your reader. Keep your chapters in one character's viewpoint. If you must switch POVs do it at a scene break. Ease back on the intertwining plots. If your reader can't follow the story, they will close the book. It doesn't have to be complicated to be interesting.

5) Disliking the characters. It's okay to have a flawed hero/heroine. Just don't go overboard. If I don't like the players, I won't read your story. Period. Don't fall into the trap of creating a TSTL female. Too stupid to live. There's a crazed killer who just broke out of the mental hospital that abuts her property. Your heroine thinks tonight is a great night to camp out on the lawn with the kids. ((sounds of books slamming shut and hitting the wall.))

6) TMI. Too much of anything will spoil your story. If it's a romance, keep the romance alive with sexual tension. Too many bedroom scenes become boring and don't make the story progress. If it's a mystery, don't just have murder after murder. Give us a clever mystery to solve. If it's an action/thriller, don't have chapter after chapter of fight scenes and bombs and car chases. The reader needs to get a break. And this is the perfect spot to talk about cuss words. Use them sparingly. Okay, gang members swear. To be realistic you have to have dialogue that fits. Just don't overdo it. It is very easy to lose readers from extended profanity.
7) Avoid repetition. If you've told us about your heroine's insecurities with men, don't tell us again another three or four times before the story ends. We get it.

8) Typographical and grammar errors. Don't edit your own work. You won't catch the little errors. Your eye tends to fly right by misspelling s because you know the story. Find a critique partner and swap work. If you're not sure of a tense do some research. "She are pretty" does not make you look like a professional author to the reading public.

9) Factual errors. Whatever it is that you're writing about, someone out there will be an expert in the subject. If you don't know medicine, be careful when writing a hospital scene. If you don't know police work be careful when writing about a crime scene. If you have your characters doing something dumb, not only will you get letters, you will lose fans. These types of errors take the reader out of the story. That's not where you want them. I read a story once where the victim supposedly lost over two and a half gallons of blood. Really? A human being only has six quarts (5.6 liters).

10) Too helpless and too hopeless. Don't have your plot revolve around something that could be easily cleared up by asking a question or two. Let's say your whole book revolves around a misunderstanding. The wife saw the husband with another woman and assumed he was cheating. She never asks him about it. The reader just goes from chapter to chapter watching the wife acting hurt and acting out, while the husband hasn't a clue what's going on. This plot is too thin. The whole thing could have been cleared up with a few questions. Your reader will be rolling their eyes and screaming ASK HIM ABOUT IT FOR PITY'S SAKE!