I had DK as a main through LK, Cata and MoP. But recently choosing new classes because we have boosts.
Current main is a warlock. He is actually a MoP boost from when they first came out... I just wanted to do the Green Fire questline... he sat unused for 4 years.
Starting the expansion I was going to be a Warlock, Holy Pally or Resto Druid... I wanted a ranged class that would be picked up easily for pug raiding or core to a guild roster.
I primarily raid. Warlock is nice because the raid needs healthstones and a battle rez. Its always a powerful ranged class. Everyone wants summons, especially pugs... its easy to get groups on bosses you have no business in because people want you to summon.