Authorial Avarice -- a belief that story length measures story worth, and the longer the better, resulting in enormous rambling novels lasting for upwards of fifteen hundred pages in print. Ultimately said novels are only good for two things: killing bugs the reader swats or serving as an emergency doorstop while unloading groceries from the car.
Book Bitterness -- producing envy-riddled text, in which the author savages some colleague whose books sell better, a highly successful personage with more money than God (usually Bill Gates), or a reviewer who hatchet-jobbed one of their books (for chronic variations, see David Drake/Charles Platt Syndrome.)
Elemental Indolence -- to refrain from properly researching/crafting/rendering some aspect of or element in the story because the author perceives it to be too much work, or something the reader (an idiot) will never understand anyway so why bother explaining it? Usually results in a lot of telling versus showing, housekeeping dialogue or the employment of the word chiaroscuro more than five times in the story.
Literary Lechery -- the author's inclusion of explicit sex scenes or other gratuitous and shocking material, not because they actually serve the story, but in the hope of boosting sales or because their editor told them too. If the author becomes a Christian fiction writer later on in life, s/he will tearfully apologize in public for this (see lesser sins, Hester Prynne Hysteria.)
Over-Ornateness -- a case of extravagant word gluttony, when the author spends most of the story describing things while employing at least three flowery adjectives for every other noun. Also called purple prose, usually begins on the very first page of the novel, when the story initiates with a florid variation of Bulwer-Lytton's notorious opener It was a dark and stormy night . . .
Voice Vanity -- an author who makes all of their protagonists thinly-disguised, highly-idealized versions of themselves; the protagonists are inevitably beautiful, smart, handsome, thin, tall, rich, universally admired and everyone in the novel wants to sleep with them. Usually touched off by the author's messy divorce or overnight success with the first book.
Writer Wrath -- when the author's anger over a political, social or other real world situation takes precedent over entertaining the reader and results in visible ax-grinding within the story. Warning signs: characters who are obvious spoofs of unpopular public figures, wars that result in the unsavory demise of a spoof character, any evil overlord antagonist named something like G'eor Gewb Ush.
Feel free to add your deadly writing sin in comments.