Field must contain all letters of the alphabet.
Maximum word length: <n> characters.
Field can contain any word only once.
Sentences must (/not) exceed <n> words.
Word count of every sentence must be a prime number.
Every sentence must be <n> words longer (/shorter) than the previous sentence.
Word count in every sentence must be the same as in post title.
Every sentence must contain a number.
Comment must not contain any (/must contain all) words from the post title.
Post/comment must contain an assertion of who we are at war with, this week.

All of course also for comments.

And yeah, we were always at war with Eastasia … just to make sure.