- Are used as an excuse for appalling infodumps. Huge quantities of information are crammed in with very little attempt to craft an actual story.
- Depict a long-ago historical event the author wrongly thinks we need to see up-front to understand the story.
- Feature a generic evil overlord being generically evil. He burns villages, he tortures people, he kicks the dog. Yawn.
- Attempt to be ominous yet unspecific by using painfully cliched language of foreboding: "Something terrible is coming." / "Yes, and a hero must rise to deal with it, but I fear he will succumb to the madness in his soul." (I don't understand why people ever bother with ominous vagueness when it's so obvious they're referring to the protagonist.)
- Show the special events of the special protagonist's special birth. Apart from the cliche, this supports the tired idea that everything special about a person is inherited from their equally special daddy. Alas, poor meritocracy, we knew ye well.
- Are completely unnecessary.
- Try to persuade us to get emotionally invested in the characters, even though we all know that prologue characters won't recur - whether they die at the end or are historical characters or aren't important at all except for the special baby.
- Don't even bother trying to persuade us that there's something here worth investing in emotionally.
- Tell us things that don't become relevant for another 80,000 words, by which time we've forgotten what happened.
- Are obvious ripoffs of other stories.
- At the end, rip the reader out of that scene and dump them into chapter one, frequently with absolutely zero continuity of time, place, plot or character. (I complained about this in my review of the pilot of TV series Sanctuary.)
- Prologues cannot contain more than 50 words of exposition at a time. Period.
- Special protagonist's special birth prologues are banned.
- Generic evil overlord prologues are banned.
- Prophecies? Totally banned.
- Ominous vaguery is not interesting.
- Readers need some continuity. The prologue and first chapter should be obviously linked: the same characters, or the same setting, or the same subjects, etc.
- At least one named character with an actual personality must feature.
- Thinking, feeling, angsting, expositing, world-building, reflecting on backstory and navel-gazing of all types must be balanced with dialogue, action and character interaction.
But when I reread the prologue of Best Served Cold, I remember how enormously I love it. :)