I did a serach on these forums and I couldnt find anything so I thought I could post the results from a few experiements I performed today with the help of some friends.
It details the threat generating effect of the debuff effects associated with Thunderfury.

Thunderfury threat generation

As I’m sure most tanks who have either played with a thunderfury or played with someone who has a thunderfury understands the proc generates threat. There is however a question which I have not seen anyone answering, and that is how much threat it is generating.

The TF proc effect can be divided into 3 categories.
1: The damage effect, each time the proc activates the target is hit for 300 damage
2: The speed reduction effect, each time the proc activates the target receives an attack speed debuff.
3: The nature resist reduction effect, each time the proc activates the target and targets surrounding it has its nature resistance reduced.

As we will soon conclude each of these effects generate its own threat.

Using a tank with defiance in defensive stance the damage effect will provide a total of 391 threat 300*0.9*(1.3+0.15) = 391.5
This threat is recognised by KLHT (the threat meter used in the experiments) and is accurately calculated.
The other two effects however do not display in KLHT and that makes us able to determine the threat values for those effects.


The test was conducted with 2 warriors, 1 with thunderfury and 5/5 defiance talents and one without thunderfury and without defiance talents.

The first order of business was thus to make sure that KLHT works properly with defiance.
To do this we needed the TF tank to equip another weapon and do a test which measured the threat level when aggro is transferred from one tank to the other.

We tested this by having the first warrior tank a hederine slayer up to 15000 threat at that time the warrior targeted himself and ceased to attack the target. Now the second warrior engaged the target and built threat until gaining aggro.
The second warrior should gain aggro at a total of 16500 threat (15000*1.1 due to having to generate 10% more threat than the current target of the mob to force an aggro switch) which is exactly what happened.

Knowing that KLHT worked properly for a non TF scenario we could start the real testing.

The first step was to calculate the threat gain from TF on the attacked target.
This was done by the non TF tank tanking a hederine slayer up until 20140 threat and then selecting himself and ceasing to attack the target. Now the TF tank could engage and keep building threat until gaining aggro. Since KLHT recognises the damage effect from thunderfury aggro should be gained at 22154 threat if the other debuffs had no threat generating effect. This was not the case however and the TF tank gained aggro at 19700 threat.
By subtracting the displayed threat from the real threat and counting the TF procs we can calculate the threat generation from a single TF proc.
Real threat > 22154
Displayed threat = 19700
TF procs = 7
Threat per TF proc > (22154-19700)/7 = 350.57 in defensive stance with defiance.

This can be compared to sunder armor giving 377 threat in defensive stance with defiance.
With a proc rate at 25% this constitutes a significant threat increase and its no surprise that mobs tend to stick like glue to tanks with TF. It also raises the question if its viable giving rogues TF due to the massive aggro gain that undoubtedly is better used for a tank.

These 350 threat are however the result of two different effects, the speed reduction and the nr reduction effect. To find out how much each of those two effects contributes we made another test.

The nature resistance reduction is a multi target effect hitting a total max of 5 targets.
We used this by having the TF tank gather up 3 shardtooth maulers and slowly killing two of those with auto attack only leaving the third mauler untouched. This third mauler had during the course of killing the first two received 8 NR debuffs. We now let the stripped down second tank (in the nude in order to maximize precision in our measurements) beat on the bear until gaining aggro. A total of 1900 threat was generated before aggro was transferred to the new tank.
Since aggro was transferred at 1900 threat we know that the threat generated from the 8 nature debuffs was a total of 1727 or 216 threat per proc with defensive stance and defiance.


Conclusion:
The Thunderfury proc generates threat in addition to the threat generated by its damage.
The attackspeed debuff on the main target generates 134 threat.
The NR debuff on the target and surrounding targets generate 216 threat.
Threat values used are with defensive stance and defiance and defined by KLHT with sunder armor giving a reference threat value of 377.

What we haven’t tested is if these debuff effects are modified by the defensive stance modifier or the defiance modifier.

Interesting finds, we only got our first TF a few weeks ago after a year and 3 months of MC, so don't have too much experience with it yet. If this is true though its threat generation looks pretty insane compared to anything else.

Never used the addon so please remind me, does 1 threat in KLHT equal 1 point of damage?

1 threat is 1 damage without modifiers yes.

However several classes have modifiers to their threat generation. For example warriors in battle and berserker stance have a 0.8 modifier while in defensive stance a 1.3 modifier.

Yea, I'm aware of that, we did our share of threat testing back when BWL was released. Haven't ever used the addon though and it's been so long I've forgotten the threat values for each skill, and I recall the Nurfed threatmeter had 1 point of threat equalling 1 point of healing done so I asked just to be sure.

I'm a little slow on the maths and its late for me, is the threat component of the slow debuff and NR debuff (134 + 216 threat) applied in full on every proc in addition to the 300 damage (350 threat) for a total of 700 threat per proc? (assuming def stance and defiance).

It is in additon yes (however the damage is 390 threat as 270*1.45=390) totaling the threat to be 740 per proc.

So, baseline values (before defensive stance and defiance) for the threat looks something like this?

Damage: 300
NR Debuff: 149
Speed Debuff: 92.4

With those values, assuming my math is right and you were taking def stance and defiance into account, and the .8 threat modifier on rogues/fury warriors then the threat per proc for them is 240 + 119 + 74 = 433.

Nevermind my first post.

Those numbers are pretty impressive, I´ve always thought that Thunderfury is a vastly underestimate weapon and it seems I was more than right.

And if I'm not mistaken, the threat it generated was nerfed once? (not just the procrate, but the amount of threat from the actual proc)
If so, then I dont dare to imagine how insane it used to be.

This is a little surprising because qualitatively for as long as I've had the weapon I really didn't think that the proc generated any bonus threat other than the usual defensive stance/defiance modified value. After the somewhat recent change, the debuff seems to generate a small amount of threat -- but that's about it.

Actually I think the threat was buffed, the part that jumped to nearby mobs didn't cause threat until recently afaik.

Awesome work Brissa! Could you tell me what the proc looks like in the combat log, is it just "Your Thunderfury hits x for 300 Nature damage.", or does the proc have some other name?

The message is "Your Thunderfury hits X for X Nature damage." Remember the proc can crit and it can be reduced by Defensive Stance.

This certainly explains why flurry helps thunderfury tanks so much. If you have flurry up half the time (takes 14-17% crit depending on abilities used), thats 15%more thunderfury procs. Thats a big boost. Our MT is 0/32/19 with 5 defiance, 5 toughness, imp shield block, last stand and he holds agro like glue.

I got called silly for thinking TF procs had extra aggro from the debuffs.... go go actual testing. Now we just more bindings =|

Yes it has

I definitely knew it had aggro to it, me and a friend were farming one day hes a fury warrior and he proc'd Tfury like 5 times in a row on a grouping of mobs, i then proceeded to hit on of the mobs @ 100% hp for rough 1000 frostbolt and it didnt come over to me, i was dumbfounded at the time and came to the conclusion it had to have some aggro generation.

Great post.

I dont recall when the patch was released but tests were done the same day that i posted the OP.
As for seeing if the procc still generates aggro just go hit on one mob adjacent to another. If after a procc it takes more than 1 damage to force the mob not being hit on over to the non TF wielder it still generates threat.

I'm currently writing threat values into my dps warrior spreadsheet, I'm trying to understand stances and how they affects Thunderfury(and HS). Couple questions:

1. Does stance agro modifiers affect the static threat bonus from HS, similarly the static threat from the Thunderfury debuff and slow, and the threat from generating rage?

2. It is mentioned that Thunderfury can crit, what's the determination for that? Spell crit?


Thanks

From what i understand Def stance affects all the threat you do. Meaning the TF procc would do more threat in defensive stance than in battle stance just like the HS threat bonus is.

The TF procc can crit but from what my TF source has told me its a relatively rare occurance (absolutely not melee crit) which would suggest its actually spellcrit.

Well, you have to take the rage generation into account (right?).

I'd heard the same thing, and our MT has less of an issue of picking up 5-6 mobs around him with TF + WF than he used to, but that might be because our other tanks/hunters/whoever are getting faster at adapting.

Deathwing: I highly recommend studying Kenco's threat meter code. He has figured out which threat modifiers are constant and which are a multiplier. Take a look at KTM_Data.lua for the values of various spells/abilities and KTM_Combat.lua for how things are added/multiplied together.

As far as modifiers go, its going to get very easy in the next patch. Just multiply all the modifiers together and apply it to all threat generated. For example, Beserker stance + Fetish = 0.8 * 0.3 = 0.24 so you would generate 24 threat for every 100 damage you did and heroic strike would generate 175 * 0.24 = 42 bonus threat (on top of the threat from damage). On the other hand, defensive stance + talent + threat on gloves would be 1.3 * 1.15 * 1.02 = 1.525 so that would be 152.5 threat for each 100 damage and 175 * 1.525 = 266.9 bonus threat.

I would also assume that the TF proc's critrate is based on spell crit, but an easy way to find out would be to get the dragonslayer buff and go grinding. The healing aspect of BT and heroism card crit at about 10% so if TF's proc crits at about 10% too then there's your answer.