1. It turns out there is no one single "way black people talk." So you can't throw in Southern Urban African-American, Louisiana Creole, Caribbean, and I Swear I Heard A Guy Say This On The Bus into one character within four pages without the person deliberately imitating anybody else and have your readership nod sagely and go, "How vividly urban," Whitey McWhiterson. Even if your readership is as white as you, it turns out some of them will have met actual black people! Personally! And heard them speak! Also, going straight to the absentee black dad trope does not make you groundbreaking and edgy. Particularly in combination with the black youths in gangs trope. It makes you...well. Let's say it makes you not a writer I want to read, whatever else it makes you.
2. It turns out that historical personages from non-European countries do not always think that Europe is the be-all and end-all. So having your non-European protag go on, in the first person, about what awesome things are happening in Europe in their time period--things that will be Sir Not Appearing In This Book--is not really very realistic. Or interesting, given that history happened elsewhere also.
3. Stop wibbling! If it might have happened this way and it might have happened that, do something meta with it, but don't just drone on about how you're not really sure. You're the writer. Be sure.
4. Stop wibbling! If your character spends the first five pages whining about how she can't make up her mind and doesn't know what she wants, the reader may decide she doesn't care if the character gets what she wants, or what's coming to her, or a bright blue lollipop with a bow on.
5. When you are already describing demons, layering adjectives and metaphors to tell us how really truly darkly demonic these demons are will not intensify my sense of forboding. I will not think, "Oh no! I had hoped that our heroine might be menaced by a dim demon from the shallow pits of hell! Not a midnight black darkety dark one from the deepest pits of hell dark dark scary ooh!"
6. If your protag is a jerk, they had better be an interesting jerk fast. I don't have to like all the protags. I do have to want to know what happens or care about another character or something enough to overcome the "wow, this person's a jerk" reaction. This can happen. You just have to work for it.
6b. All right, your protag is a lovable loser. You forgot the lovable part. Done now.
7. Suspension of disbelief has its limits. "But that's impossible," can be overcome much more easily than, "But that's stupid."
8. Kids These Days will at some point be the right age of people to read your book. Heaping scorn upon them for not having the totally wise and awesome generational conventions of your generation is not going to make them adore you. In fact, very few things date a book faster than the certainty that the current generation of young people is wrong about everything.
9. I know the temptation of Gratuitous Capitalization. I do. But resist. Seriously.
10. I can do without a plot. What I can't do is do without a plot when you've got a plot. If your plot is, "And there was totally obvious innnnnncesssssst"? You fail.
Wait, I didn't quit reading that one. I'm looking at you, Antonia Susan. Your novella is bad and you should feel bad. Sheesh.