Heya all
Just wanted to point out something it's possible you missed - P10 AV has more specifics on how double weapons work :
Wielding a double weapon is like wielding a weapon in each hand. The first die given in the damage column of the table for a double weapon is for the primary (or main) end of the weapon; the second damage die is for the secondary (or off-hand) end. You can use either end of a double weapon to deliver an attack unless a power specifies a main or off-hand weapon attack.
Summary of rest : Magic effects such as jagged, etc only apply to the primary end, but the enhancement bonus applies to both ends.
Higher level characters, therefore, may end up shying away from double weapons in order to have two weapon effects (one from each hand). Granted, you gain the enhancement bonus for the second end without paying for it... but min-maxers are going to scorn that for a second weapon property on each attack, if they have the cash at hand.
Also, on the topic of double-weapon believability : Batleth
I think the easiest fix for double weapons works thusly (and my vote-worthy suggestion, if I am allowed one) :
1) Remove the defensive ability from double weapons. Now it is just higher damage with off-hand in exchange for a feat.
2) In the case of the Double sword, the primary side is heavy, the secondary side is light. All effects with the heavy blade keyword have to use the heavy (primary) side, all effects with the light blade keyword have to use the light (secondary) side.
On the subject of Heavy Blade Opportunity and sneak attack, as your 'at-will' opportunity attack ability would trigger using the heavy side of the blade... rogues are unable to do it (no heavy blade at-will attacks).
The other suggestion I have is to vote on the 'Appendix #1' Chapter immediately, as it seems nobody has an issue with any of that stuff, and then amend the 'Equipment' chapter as necessary to get it into play immediately, leaving the 'Magic Items' chapter for the last, as it will take the longest, and have impact on fewer characters.