As for tanking with the DK for any team... it's just another option. For example, unholy tank with anti magic shield... drop shield over clones on caster boss? Also I think the unholy build will be the aoe tanking build. I think Blizzard has done a good job on warrior tanks (my main tank on my first team). Pally tanks are insane for boosting now (no healing or drinking?). I don't have a drood tank (yet) so I can't comment on it. I do think at 80 niche tanking roles will become apparent in later raiding content.

The DK starting area is not really MB friendly, but can be managed. As for the masses of DKs on day one / week one, remember the DK staring area is phased so it might be better than Borean Tundra or Howling Fjord land rush. Unfortunately the DK starting quests are over way too fast. Utilizing RAF, if possible, will most likely put you in Outlands at 60 or higher.

As for PvP melee classes will fall fast to a team of 5 DKs, if you can keep them in front of 5 swinging two handers long enough for * strike, death coil to hit. I think Casters can be handled with a nice alternating rotation of death grip and ice chains. MBing DKs will not be easy but should provide a fun challenge. More than likely people will mix the builds in DK teams (2 blood, 1 frost, 2 unholy), I have not confirmed if presences stack, but with auras doing so now... it could be sick.

I have played DKs a bit and as a solo toon they are a kick in the pants, and I think done right they will be crazy for a MB.