The way you could see it as for DK's I would think is two ways when it comes to the WoL reporting.
1. The amount of dmg taken from the boss in relationship to the raid.
2. The amount of dmg the boss does to its targets.
The idea around all DK's can tank maybe true but the boss can tell who is tanking. When the boss hits a target for above a set % one would think that it is being tanked by that person(s). When you see damage taken on 2-3 targets is double or higher those are the ones tanking the encounter not the dk's taking the normal raid dmg. All i know is WoL is still in the phase of getting all the information they want out of the meters. Give them time and they will soon be able to show it to us.
I got a reply from the guys at WoL detailing how they pick each DK spec. They basically look for a certain amount of say, HB and FS usage would be a Frost DK. I am asking if he could find/pull me some data with say a Frost Tank being FS/HB and RS.
You have to be avoiding direct attacks on you use rune strike, so I'd say anything more than a few uses of rune strike will clearly indicate a tank. If you wanted to break it down into the different specs, you'd have to look for signature abilities. e.g. scourge strike = unholy, howling blast = frost, heart strike = blood. Since you can't use those abilities unless you have specced fairly deep into the trees, frequency of using them isn't that significant... ie. if they can do a scourge strike they must be unholy spec at the time.
Well it depends if you really want to break apart the 3 different specs of DKs.
If you don't, it's purely RS usage, if not, add HB, HS & SS to them.
Personally I wouldn't mind seeing the 3 specs pulled apart, see what differences there are in a single class. But even so, think it's fair to assume that all tanks have RS damage above say ... 30% melee damage?
TBH the majority of the remaining dk tank population (at the higher end of progression) is still running a blood spec. I wouldn't say frost is irrelavent, but if we're gunning for a general picture of dps @ dk tanks, blood is a safer bet.
if the purpose of the thread is an across the board tank comparison, no need to waste time refining the specifics of one single class, just going by high RS frequency to sort the tanks out and go to town