1) Show-offy prose
See, this is why I have a problem with these sorts of lists. What if you, as a reader, sometimes enjoy descriptive passages? What if you appreciate beautiful or stylistically bold prose of the kind that Patricia McKillip or Jim Crace write? Then you're faced with the prospect of writing for people who have dramatically different tastes and expectations than you have yourself. And isn't there something cynical about that approach to writing?
2) Head-hopping
If third-person omniscient requires a masterful writer to pull off, then what does that say about fiction from decades ago, back when third-person omniscient was the norm? That every author back then was a master? It seems to me that modern writers today, especially younger ones, may be ill-equipped to write in third-person omniscient if they've read little of it themselves. But steering new writers way from it altogether seems wrong-headed.
7) Mary Sues
Actually, in the SF/F and YA genres, Mary Sues seem to be the norm. In fact, you don't seem to have much chance of finding popularity in those genres unless the protagonist is an amazing ass-kicking ass-kicker who is amazing at everything.