Being Dk means:
You are hybrid but still you can be happy cause nobody can force you to heal.
You can be specced as tank and in average pve aoe encounter you could do dmg more then average hybrid
You have 3 talent tree which all can be your tank or dps tree
As DK you have posibility to use best dps enchants for free
As Dk you can say to every1 how imba are you and pro in Arena no matter that class exist only for 5 months.
As Dk you can insult others how they dont know to play no matter that you reroled cause of frustration of your playing on previous class(read as: Priests/Warior/Druid/Mage/Lock-> classes from most rerolers come)
Thats short list why to rerol Dk and reasons why not rerol... well cant find any atm except if you really like your class and wanna understand his mechanism of playing in pvp/pve no matter how much it sux in comparison with dk