There are a few changes to the way damage bonuses are applied to a character which will be available on preview soon.
The first change is that Critical Severity, Combat Advantage, and Damage Bonuses are now in separate categories. What this means is instead of all of the categories adding together and then multiplying off of the base damage, they each multiply on top of each other. A simplified equation of what is happening behind the scenes is:
Final Damage = Base Damage * Critical Severity * Combat Advantage * Damage Bonuses
The second change that goes along with this is that Damage Bonuses are added together before multiplying on the base damage. Previously some damage bonuses added together and others multiplied with each other. This created a situation where different builds/classes were ending up with different multipliers. Now damage bonuses will universally add together before multiplying off of the base damage.
This should create a method of damage increase that is clear and consistent which will make planning out the setup of a character easier.
EDIT: With these changes, Vorpal was reduced because the effectiveness of Critical Severity has increased and to bring it more in line with other weapon enchantment options.
Further edit, looks like the Vorpal change won't be in the preview update, at Rank 14 the crit severity bonus is now 20%