I argue that a bad post is one of two things: bad content, in which the actual formatting is hard to read or its meaning is unclear or perhaps the post is over-short or over-long; or the other bad post is one of bad fidelity. That being a post which either disregards the setting or posts something really odd/out of place for the character...etc.

One of my personal favorite posts (forever lost to the recesses of the old RPG) was a post about a flawed, mute barbarian character who communicated by stamping and growling and grunting...etc. The character did something that, in hindsight, was foolish: but the way it was posted was fantastic and it sort of solidified her as the 'stand out' character. If she had say, just spoken, the post would have been much less foolish, but it would have been a bad post.

/my2pence.