The Secret Ruleset
In addition to the publicly available main Ruleset, there are some Rules which aren't official but are nonetheless enforced. This alternate ruleset exists because us players don't usually play by the Ruleset as written, but instead play by our interpretation of the Ruleset. Also, the Blognomic community has its own standards for conduct. New players, however, often miss some important unspoken rules.
The important unspoken rules which I know of are as follows:
1) If an unwritten rule conflicts with the Ruleset, the Ruleset takes precidence.
2) An action is illegal if a CfJ challenging it would pass, or if most players believe it to be illegal.
3) Actions based on an interpretation of the Ruleset not obvious from a casual reading may be illegal even if strictly permitted by the Ruleset.
4) Large-scale spamming is prohibited. About the most you could get away with is the posting and Self-Killing of 3 proposal blanks. However, the exact limit depends on how good a reason you have for spamming. - Now covered by Ruleset
5) Illegal actions may be reverted immediately even if the reversion would also be technically illegal.
6) Scams based on the properties of EE are strictly forbidden. - Now mostly covered by Ruleset
7) Scams based on the core ruleset are strongly discouraged except in cases of emergency.
8) A player may combine several actions into a GNDT entry if they could legally have been performed separately. - Now covered by the real Ruleset
Of these rules, 2 probably has the greatest implications. For example, whether an action is legal or not often depends on who performs it, the results, who writes the CfJ if it gets CfJed, and perhaps how soon the player who performed the original action responds to the CfJ.
NOTE: The Secret ruleset is more what you'd call "guidelines", than actual rules. Another Note: This is the work of a single player. Not all players agree with these "rules", which are not binding.