The "reputation" number on a post is the result of a process that tends to correlate with a few distinct factors, yet also displays an appreciable degree of randomness.

The strongest factor is whether the node happens to be in a thread that gets moved to the PM "front page" (the Monastery Gates) -- this determines the number's magnitude. Next is the quality of the post itself, which often (but not always) determines the sign (positive for clear, concise, goal-directed posts that show evidence of a working brain; negative for posts that are vague, cumbersome, snide/pouty/unpleasant, or simply brainless in one or more myriad, possibly obscure ways). This latter effect is weak -- prone to be masked by the inevitable random component of the process -- when the primary effect (front paging) does not come into play.