Is there any extra benefit to added elemental damage on weapon attacks? Are certain creatures vulnerable to certain types or do certain types have certain effects (such as shock damage stunning)?
Yes, here are the damage types in the game and their effects. From IGN's game guide:
(The grey parts are from IGN)
Any Wrath abilities used from the Necromancer Tree are more powerful the more Arcane stat you have as well. Strength affects Harbinger Tree attacks.
I have done testing in game and have seen these effects myself.
Ooooook. The list is like this:
The first ones are the first to encounter and weaker versions, having small effects:
Now as for the upgraded versions, they all carry the same effects of the juvenile versions, but add a few extra effects and goes as it follows:
Also, since someone mentioned Arcane and .... got it wrong ... even if it doesn't directly answers your question, I`ll cover that aswell here: It only affects the damage of the skills in the Necromancer tree and Strength is what affects the Harbinger skills -- you can test that easily by equipping and unequipping a str or arcane item and hovering the skills in the menu to check what changed and by how much.