There has been a lot of discussion here and elsewhere in regards to damage and how it works in regards top combining damage types, per-combined damage types and how it all relates to vulnerability etc.
I picked up the new Essentials Rule Compendium today and on page 114, the first thing stated under Damage Types:
"If a power has one of these keywords [see damage keywords, above with descriptions], it deals the associated type of damage."
This means that as some of us assumed previously, adding a damage type to an attack infers the damage keyword and thus the rest of the attack deals the associated damage type - adding 1 point of cold damage makes the entire attack deal cold damage, as well as any other type of damage being dealt with the exception of specific instances such as the elemental weapons, weapons that specifically deal half of a type and powers such as Blazing Starfall (Although technically given this rule the entire thing is Radiant and Fire... the general doesn't quite get around the specific.)
So, we can trigger multiple vulnerabilities (which we knew) but also, an enemy (or player) must resist every type of damage being dealt in an attack in order to resist any, and then only the value of the lowest resist counts.
This, obviously has an impact in terms of Optimization as we know have a definitive answer.
Thoughts? Does this make anything much better, worse?