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-No Multiclassing penalties. They're just annoying and play heavily on stereotypes.
-Skills involving social interaction are out; any interaction with NPCs is Roleplay-based.
-Unarmed attacks deal lethal damage, threaten surrounding squares, and do not trigger AoOs. Concequently, Improved Unarmed Strike does not exist.
-No godslaying. Sorry, but you can't stab sentient metaphysical concepts in the face.
-Aid Another doesn't do anything. Mainly because it's too bloody vague. "I aid him." "How?" "By AIDING."
-Withdraws and 5-foot steps trigger AoOs like any other movement action within threatened squares.
-For starting ability scores, players roll 5d6b3. They may reroll as many times as they like, but they must reroll the entire set each time.
-RAW is always secondary to Rule 0.
-Cross-classing must be done in-character. For example, if a rogue decided to become a fighter, he must seek out a professional trainer or join the military. He can't simply pick up a greatsword and call it a day.
-Penalties from heavy armor and heavy loads stack.
-Greataxes deal 2d6 damage, not 1d12. Similarly, Scythes deal 1d8 damage, not 2d4. This is to keep scaling consistant.
-No Use Magic Device. You cannot "fake" magic.
These are just in general, mind. Campaign-specific rules get alot more convoluted.