3 or more DKs

I am thinking of starting a DK team. Question? Do the Presences and diseases stack or is there synergy?

presences, no.

diseases, yes, mostly. Each DK can have their set of fever fever and blood plague, but the spec'ed debuffs like ebon plaguebringer, icy talons, and chilblains.

Runetap glyph does stack, which is all the rage.

i would recommend 1 unholy/blood dk(blood up to runetap/imp runetap for more heals), 3 frost/blood(blood up to runetap/imp runetap for more heals) two hander dk spec as it works better given what talents we snag in blood tree, and a shaman healer or pally healer, as the dks should have most of the aoe healing covered and an all plate team is fun(you could theoretically spec the pally ret/holy with divine storm healing and spot healing when needed)

if you're only doing 3 i suggest doing 3 frost/blood dks Armory Moocrewa, Moocrewc, and Moocrewd on azgalor server. i run a team of 3 frost/blood dk and a feral druid tank and i mow thru stuff pretty well, and its easy to set up the macros and such

I am planning on 4 DKs and a tree/ or an 80 DK to tank/boost. Got two through the start and thru the Dark Portal. Will work more this week and then box them a little this weekend.
I will look at the builds as I have just picked what sounded good. 1 is blood and one is unholy. My 80 DK is Frost/2hd. The tree is 65 so will help keep them all going when the self-heals fail.

Im using a Blood DK as a tank for my team (DK, Sham, Huntx3) and its really easy to have a high DPS single spam macro going on them. Just starting to creep into the frost tree (Level 63 now) and I can imagine having 3+ of those things would just rip everything apart.
With 5 of them all blood spec you could almost get away without a healer, each pull rotate your presence so that the former tank is now in blood spec and getting a whole lot more heals. Or hell just leave them all in blood pres unless it gets really bad.
So many possibilities to try out.

I've tried starting a fresh team of DKs on a new server to run with a friend of mine. Running 4 and keep switching up the specs to see what I like better. Been a while since I played them, but when setting up your dps macro, make sure to cast your diseases then spread them first before you land any hard hits or else you'll be taking down mobs without them up. Always go for the tanks first then the casters in these cases too. Usually the other way around, but its easier to mow them down if you can get your team to spread lay them diseases on the strongest one first then spread them. I've done this in Ramparts and the whole mobs, both warriors and casters went down at roughly the same time. Casters pretty much died from diseases instead of me smackin them around. Bloodworms are also great for healing yourself, but ineffective if you're not critting enough for them to pop. If you can, I would have a Ret Pally in there also for spot healing and casting DS in your dps rotation. My main team is in my sig and that's how I roll. :D If you get the Anti-magic shield, then put it and Death Grip on Round Robin.

I'm running 3 Unholy DK's, they are 80 and I love to run them with my priest. One has a Frost spec to tank instances. I like the utility of unholy as it can really take advantage of many ranged attacks and as far as I can see it's ruling the DPS meters for this patch.