The DK starting Area takes place before WotLK and the battle of light's hope chapel occurs right before WotLK begins. It's instanced off to provide a private time zone of sorts for the DK's. If you look around you can find a bunch of other Naxx bosses.

I'd like to see Taelan Fordring as the fourth horsemen, it would have a heavy helping of lore attached and he fits the bill of a future Death Knight.

It is revealed what is beneath Light's Hope Chapel or at least the name of the mysterious... thing. It is known as the Light of Dawn which is most likely a relic, urn, or talisman of some sort that completely negates the unholy.

My biggest question would have to be; Why is Arthas the "final" boss of WotLK? From a lore standpoint an Aspect would be much stronger than some Death Knight who commited Patricide, threw on some magic armor and grabbed an Artifact weapon. In the War of the Ancients, even after transfering a part of his power into the Demon Soul and after the Well of Eternity's use was locked out, Malygos used his power to erase a piece of existance with a wave of his claw. Now if being erased from life doesnt one-shot a tank I dont know what will.