Diablo III – How Blizzard Determines If a Retail Copy of Diablo 3: Reaper of Souls Was Preordered

As of a recent beta patch, runes are no longer items that are dropped. Instead they are basically skill modifiers that are unlocked as you level up. Each time you level up, you will unlock new skills and/or rune choices that affect your existing skills. You choose a certain number of skills, and a corresponding rune for each, to equip as your skill set. Each skill can have up to 6 different rune versions of it, but you must select which one you want to have equipped at any time. There are no longer different levels of rune abilities, it is simply a matter of having it unlocked or not.

I have. It's pretty simple, really - it's double dipping. You get its effect twice.

First, it applies a modifier to your weapon damage - possibly unique, I haven't seen any other sources of a modifier like this - which is used in its own place in the calculation formula.

Second, it adds that same modifier to your skill damage bonus. This is why you'll notice your skill damage go up by 10% (15% with force weapon) in your details page of your inventory. So what ends up happening is magic weapon adds 1.1*1.1 = 1.21 = 21% bonus damage, instead of the listed 10%. (32.25% with force weapon rune)

The weapon damage modifier stacks multiplicatively with other sources of skill damage bonus, such as glass cannon and sparkflint - which all stack additively with each other. Glass cannon, sparkflint and force weapon combined would give you 15% + 12% + 15% = 42% skill damage modifier, and then there'd be an additional 15% weapon damage modifier, which would net you 1.42 * 1.15 = 1.633 = 63.3% damage increase.

(I played it fast and loose with two equals signs for the sake of brevity and clarity.)