This is a slightly esoteric Death Knight issue, so those of you not yet converted might want to just move along. It is also an issue close to my own heart, as I am pretty enamored of my Unholy DPS DK. What am I talking about? Ebon Plague stacking, of course. The other two main DK diseases, Blood Plague and Frost Fever, stack just fine, but if you've got two UH DKs with the talent in your raid, only one of them at a time gets to have EP applied to the target. This lowers the DPS of any DKs that don't have it applied, since so many of our skills do damage based on the number of diseases we have applied to the target. Most sources estimate the DPS loss as not very high, but it's still annoying as heck.
Ghostcrawler has finally addressed this problem, and says they plan on fixing it by making EP count as a third disease for everyone that's talented for it, regardless of who cast it (if I'm reading correctly). But he does note that this fix is not top-priority, saying "our priority right now is making sure more DKs spec Frost or Blood, not how to reward groups that bring multiple Unholy DKs," going on to say that they nevertheless will fix this bug.
According to Armory Musings (thanks to the Scarlet Mathematician for pointing me to that site), Unholy does account for almost half of all Death Knights, so it makes sense that Blizz would be trying to up the market share of the other specs some. Still, I love the Unholy play style, so I hope they fix EP sooner rather than later.