There are a lot of misconceptions about how crit damage works. Some say it "doubles your damage" or increases your damage by whatever %. I believe many have been misinterpreting the phrases that wikis/other people provide. So I will attempt to clear up misinterpretations with my poor wording <3.
From what we do know:
Attack increases all base damage by 1% per point. Your skill's base damage will be multiplied by 1 plus this value.
A wizard with 500 attack has blizzard, which does 57-78 per tick. 500 attack will increase the damage to 285-390 per tick.
What about crits? No one has bothered to give a formula yet, but let's say for example that a basic, unmodified crit damage does 50% more damage.
Ok! Now Let's say our 500 attack wizard has 400% critical damage crits with his blizzard! What will happen?
Well, we know that we have a 285-390 damage blizzard, so let's just multiply this by four! This gives us 1140-1560 per tick.
Crits are pretty valuable, eh?
In all honesty, I do not think that is how crits work. I've heard people say they double your damage, so +critical damage will make it more than double!!
This is an absurd assumption. Why? Because it would make certain builds stupidly good in comparison to other viable non-crit builds.
A wiz with alabaster rune in blizzard increases its critical strike chance by 80%. With a no crit chance build, you will be doing unfair amounts of damage with just +damage and +crit damage.
1140-1560 per second with no crit chance investment just from one rune makes any other choice obsolete. Why reduce the arcane cost from 40 to 24 to do 285-390 when you could just spend 16 extra AP for 4x the damage?
It just doesn't add up, so let's assume crits do not "multiply your damage" by your increased crit damage.
This is how crits probably do work: They multiply your base damage, which is effectively just adding your bonus damage from your attack with your bonus crit damage.
Basically, our 500 attack wizarad increases his base damage by 500%. A crit would add it's bonus to that, so it would be a 900% increased damage blizzard (500 from attack plus 400 from crit).
Our same wizard critting with the new formula does 513-702 per crit. That's an 80% increase in the total damage of the hit by adding 400% critical strike damage.
Only 100% damage bonus from crits on the same wizard would increase the damage to only 342-468 from the original 285-390. This crit is only a 20% increase of the total damage of the skill by adding a 100% crit damage bonus to the base power of the skill. See what I mean?
Lastly, many skills just generally "increase all damage" like the wizard's crimson runed familiar or the barb's crimson runed battle rage. The desription for the wizard reads "increase damage of all attacks by 40% while Familiar is active." The barb "increases damage by 100%" while active. I would like to hear what others think about these kinds of skill desciptions. I personally think that these skills will just multiply the base damage of everything instead of increasing "all damage."
This can already easily be tested by an F&F player with a barb's battle rage. Get going!!
It is certainly possible that crit damage scales off of base or weapon damage rather than total damage, but if so, that would be a change from what we come to expect from that stat in games in general. Similarly, it is possible that all the places we see abilities which describe increased damage (crimson arc torrent, familiar, glass cannon), but if so, their descriptions are highly misleading. I would expect that while perhaps scaling effects which are alterations to your own damage (such as familiar's 40% and glass cannon's 20%) might add rather than multiply, that they will be applied after +attack is accounted for, and multiplied on top of it, and that debuffs which cause creatures to take increased damage will multiply again on top of that, *and* that crit damage will be a multiplier on top of all of these, when a crit actually goes off (be it +50% or +100%.)
If there are Blue posts which speak to the contrary, than please link them if you know where to find them; otherwise, I consider that both suppositions are speculative, but that this one follows most-closely the actual wording of the abilities.
In fact, short of a directly contradictory Blue post, I think I can prove my point at least about the "increased damage" abilities (and by necessary logical extension, crit effects as well.) Witch Doctor's "Pierce the Veil." 20% inc damage for 30% increased mana cost. If this is an increase on top of all other effects, that is "your total damage times 1.2," then it is a reasonable ability, forcing you to choose between mana efficiency and time efficiency; I would say that this even precludes the possibility of increased damage effects being additive, and suggests they will be multiplicative between each other. If what is being suggested as an alternative is true for crit, increased damage, or debuffs, then this ability abruptly becomes almost laughably, absurdly horrific. That, to me, is the far-less likely possibility.
First of all, I think the "increases crit chance by 80%" means that it increases based on your current chance to crit. So if you have zero (or very little), the rune does not help much (beyond the other freezing effect). But if you stack +crit and precision, then it becomes a much better option. Maybe even something you could build around. (Energy armor with the +40% precision rune?)
As for +% damage, I see three general possibilities.
Best case scenario (unlikely): all sources of +% damage are multiplicative, so having
gives 1.2 * 1.4 * 6 * 2 * 1.2 = 24.192 or +2319.2% damage. Everything makes what comes before it more powerful by its own modifier, which then repeats and you end up with way more than the sum of parts. For extra ridiculousness, each item with +% damage would be calculated separately.
Lesser case: sources are clumped together before doing the final calculation, so you get
+20% (passive traits)
+600% (attributes and equipment)
+60% (external effects: marked for death and familiar)
giving 1.2 * 7 * 1.6 = 13.44 or +1244% damage. There may be more or fewer categories, but the idea would be the same.
(Also unlikely) Worst case: everything is added up, giving a flat +680% damage. As oneoftheorder pointed out, this makes the passive skill increases worthless, and that would be an unacceptable oversight on Blizzard's part.