Earlier I justified DKs going to Outland. They have their reasons.
Or do they? After the fact I started to wonder about the timing. Are DKs getting to Outland at the same time as other players? Or are they a little bit late? Or are they so late that Kael'thas has suffered a mere setback, Illidan is dead, and the Legion has been beaten away?
Obviously the player gets there in a fresh Outland with everything alive. But what about the lore time?
A - Portal opens and we go exploring
B - Bad things die and we get epics
C - Arthas betrays the DKs and they break free
D - DKs go to Outland
Obviously A is before B. C is right before D. But what about CD relative to A and B? I'd guess that CD is after A. But is it before B?
DKs might be breaking free sometime after B and then going to a zone which exists in a time of A-B. That makes no sense.
Normally I'd shrug this off as the innate time travel between zones, that they all exists in a certain time. However this doesn't work as well here. Normal classes are created at a very early time, before anything else happens. If you happen to do zones in the wrong order, that's a side-effect of being able to move freely rather than any actual flaw in the story. But DKs start at a much later time. They cannot exist in their current form before that time. So if a DK is going to Outland and the Outland expedition was completed before they broke free, then they are going back in time not because they left the 'rails' but because the rails sent them there.
I fear WoW may be doing a great deal of rewriting its own story for no good purpose beside refusing to tweak game mechanics. The death of Onyxia by our hands, then the death of her by Varian, then by us again is one of the strange changes to the timeline. DKs may be the next; a class created after events but sent back to replay those events. Is this mere game mechanics? Perhaps. But I'm tempted to break my rule of "don't call Blizzard lazy" and say that it is only laziness and lack of creativity which would require DKs to go to Outland if that breaks the timeline. They could have gone to a Plaguelands tweaked for their specific class and designed to give them the XP/time of Outland (a simple +25% XP for DKs in the zones might have done the trick) and complete it with a second quest chain, perhaps to tell more of the story of the cooperation of the Ebon Blade and Argent Dawn, as a way to dole out a gear set which would prepare them for Northrend.