Brian: Hands down it’s the size rules (which may be changed or modified). Such a useful tool for scaling deadliness of a creature, spell, object or trap within the rule frame. I understand that many people didn’t like the math, but it really is fantastic.
Peter: This was a hard choice and I am wavering between two possibles. I love the experience rules. I first saw them in HARP and I am really pleased to see them being used as the default rules in RMU. I was using the HARP rules, house ruled into my RMC game but now I am using the RMU rules in their place. My other love is Spell Law, pretty much all of it from the completed spell lists to the use of the spell aquisition skill as spell mastery.