I just wanted to put this all in one post, and make it a bit easier to see. Some of this could be wrong, so feel free to post to correct any of it that isn't right and I'll edit it.
Much of this is based off assumptions about how something is most likely to operate in the game.
What forms of +Damage are Multiplicative VS Additive...
Monster Debuffs: ???
Critical Hit Chance: Additive
Critical Hit Damage: Additive
Attack Speed: Additive
Damage Increased by Skills: Additive
Damage Increased by Attributes: Additive
Critcal Multiplier: 1 + [Crit % + Crit Damage %]
For this equation both the Crit % and Crit Damage % are decimals.
Multipliers are represented as 1 + Damage Increase. 10% = 1.1, etc. This does not apply to Skill Damage, 150% = 1.5 Multiplier. It doesn't apply to Attack Speed either of course.
[Monster Debuff Multiplier] x [Critical Multiplier] x [Attack Speed] x [Overall Damage Multiplier] x [Primary Attribute Multiplier] x [Skill Damage Multiplier] x [Weapon Damage]
Remove Attack Speed to find the damage of abilities with cooldowns. Insert the Attack Speed of the ability to find its DPS.
Also, you can remove [Weapon Damage] and [Attack Speed] and replace it with your [Weapon DPS] in order to perhaps make the calculation easier when looking at abilities that don't have cooldowns.
As for Damage Reduction (against monsters the same level as you):
It's best represented as [1 - DR]. So if you have 25% reduction it would be [1 - 0.25]=0.75. Meaning the monster is doing 75% of his normal damage.
It is unknown how Monster Debuffs stack. I would assume that they do to some extent, but eventually become less valuable. They should each be their own multiplier regardless. If you had Bad Medicine (20%), Resolve (25%), Threatening Shout (20%), Breaking Wave (10%) and Circle of Protection (35%). 0.8*0.75*0.8*0.9*0.65=0.28. An enemy would only be doing 28% damage, before calculating Armor, Resists, Dodge or Block.
Armor DR = Armor / (50*mlvl + Armor)
Resists DR = Resists / (5*mlvl + Resists)
Dodge DR is simply equal to your Dodge Percentage. If you have a 30% chance to Dodge, you're only being hit 70% of the time. Or more specifically, you're only taking 70% Monster Damage. Dodge from gear should be multiplicative. That is, if you have 3% Dodge on a Belt and 3% Dodge on Boots, it would be 0.97*0.97*[Base Dodge]. The same goes for passives.
However, it's worth noting that Dodge becomes better the more you have of it. A 1% gain when you have 5% Dodge, is worth less than a 1% gain when you have 30% Dodge. More exactly, the former is equal to a 1.05% DR gain, the latter is equal to a 1.43% DR gain. At 90% Dodge, 1% more Dodge gives you a 10% DR gain. And finally, at 98% Dodge, 1% more Dodge gives you a 50% DR gain.
That being said, it only alters the value of each point of Dodge, it doesn't alter the DR of Dodge itself when calculating your Total Damage Reduction.
Total Damage Reduction =
[1 - Monster Debuff] x [1 - Armor DR] x [1 - Resists DR] x [1 - Dodge] x [-1] + 
Block would then be figured in on top of this. So it would reduce your damage taken by a flat amount in addition to your Damage Reduction.
This equation is interesting so that you can find how much damage you'll be taking on average, but since Dodge is not actually a percent reduction, it could be more useful to leave Dodge out of it and just know that you'll only be hit a % of the time and then calculate how much damage you'll take when you are hit. And then you would factor in Block after that.
Keep in mind that although it's possible to get very high DR, it only refers to monsters the same level as you. Once you're in Inferno, many monsters will be above you in level which means your defensive stats won't be worth as much. Therefore, it becomes more necessary to stack very high defense against monsters the same level as you.
These two calculations should give you a rough idea of your Damage / Defense.
Let me know what I've left out, or if anything is wrong...
Your displayed damage is a rounded figure. If you do 2.5 damage it should display 3 damage on the screen. Monster Health is also a rounded figure. Both of these are rounded traditionally. The damage displayed does not reflect the damage you're actually doing. Your damage is truly in decimal form, but the game rounds it for the display. This means that your displayed damage will often not add up to the deficit in the Monster's Health.
Crit chance cap was said to be 75% back when Precision existed. This may or may not have changed, but we could assume it's 75% at the moment until we learn otherwise.
Damage Reduction refers to just Armor in the Character Tab, not your Total Damage Reduction.
It is unknown if the Dodge cap will be 100% or perhaps something lower closer to 50%. The gains begin increasing quite a bit per point of Dodge after about 50%.
There may be an equation for Dodge involving mlvl, but I'm not sure of it at this point. But I would expect Dodge to be reduced somehow against higher level monsters, just as Resists and Armor are.
I'm not sure of a Block Cap, though it was 75% in D2 and could be the same in D3. However, Blocking works differently in D3 because it doesn't completely negate damage but rather works off a Block Amount. Also, Block % may or may not be lowered when moving.
There is likely a Damage Reduction cap as well. Perhaps somewhere around 75%? Not sure really.