Updated list!
1. Plagiarism
2. All "telling" or "showing" without a mix
3. Bad punctuation
4. Using adjectives like Stephanie Meyer
5. Using adverbs like JKR
6. Two-dimensional villains who are "evil for evil's sake"
7. Misuse of semi; colons
8. Subject/verb disagreement
9. iMpRoPer UsE oF CapiTols
10. "Using adverbs in dialoge tags." He said dramatically (c.f. Deadly Sin 5)
11. Utilizing a magnanimous variety of verbosities with the express intention of pronouncing exceedingly concise ponderings. (Saying little with many words.)
12. Describing things in TOO much detail.
13. Being a lazy writer (c.f. 29, 34)
14. Not breaking any of the other rules to make your writing interesting. (except for rule 1 - E.C. Edit).
15. Oh! Cliches!
16. Believing that dialogue should literally reflect conversation
17. Confusing homynyms, like "capital" and "capitol"
18. Also "there", "their" and "they're" (an abuse seen with irritating frequency on this site)
19. Comma/Semicolon confusion (c.f. 3 - and threads on whether to even use a semicolon)
20. Not varying sentence length
21. Making the MC too snarky or sarcastic
22. Mixing metaphors (never mix a metaphor, it's the end the hell on earth if you do - or something to do with being up a creek without a monkey wrench)
23. Pronoun confusion
24. Treating your readers as stupid (leading a reader to a question that you want them to ask, but then putting it in the text yourself just to make sure they get it [originally written as "thinking out loud"])
25. Overly dramatic (including pauses - I stretched this one to include all types of drama, not just pauses)
26. Run-on sentences
27. Writing to impress, rather than communicate
28. Spoonfeeding conclusions (c.f. 24)
29. Lack of precision (c.f. 13, 34)
30. Bumlicking/brown-nosing
31. Naming a character Skeeter (almost as bad as Eustace Clarence Scrubb, isn't it?)
32. When the writer can be seen behind the writing (dare I say, that especially includes self-inserts? See next three for explanation)
33. Character with no goal/no depth
34. Cardboard landscapes/vagueness (He walked into an apartment. . .)/hazy descriptions (a flower, etc.) (c.f 13, 29)
35. Cringeworthy Dialogue/making your characters speak in ways they never would (c.f. 4, 5, 10)
36. Exclamation mark abuse (c.f. 3)
37. Making a book sounds like a game, leveling included
38. Not utilizing the right word/misusing words according to their definitions
39. Constant repetition of words (echoes)
40. The cliche mirror description, or, just saying she had blue eyes and brown hair with lack of any 'real' description.
41. Writing with the assumption that obnoxious, smirking, self-absorbed, generally mean or unpleasant character behavior is charming.
42. Run-on Sentences - including overly long and complex sentences with no discernible purpose; such sentences normally do not have to be like that except for specific reasons: Dialogue; running internal thoughts (which, of course the author should try to stay away from using italics in these cases, as there's a discussion concerning whether such uses may hinder the writer from being published - a concern that many of us may have and all of us should keep in mind for future hope in publishing), whether in first person or third person points of view; and certain other situations that may come up from time to time
43. Trying to make a character's name overly cool
44. Overuse of foreshadowing
45. Abusing the thesaurus
46. Thesaurus solecism or misemployment, including malapropism (okay, deep breath)
47. Using weak verbs/to be too much
48. Writing to impress the reader rather than writing to move the reader
49. Not giving your protagonist a major stumbling block, and/or allowing your protagonist to overcome every single challenge thrown at them
50. Spelenk ewrors
51. Worrying to much about the rules instead of just writing what you feel works the best (c.f. 14)
52. Improper use of "to" and "too" (c.f. 17, 18)
53. Long, meaningless descriptions of characters
54. Quitting because your first draft sucks
55. Making token minority characters, or resorting to stereotypes or caricatures
56. Comparing yourself to the masters
57. Becoming too afraid of cliches
58. Trying to impress everyone with your writing
59. Not writing for your target audience
60. Doing something differently for the same of doing something differently
61. Bad metaphors
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Writing process -
1. Submitting work to a critique group without doing your own editing/proofing first.
2. Procrastination
3. Reading only within your chosen genre, as opposed to widely and voraciously