As Twilah said, just make a character that you want to play. I boosted a paladin to 100 (it's weird having a human as I am used to a draenei, and using the Gift of the Naaru. It is strange not having a HoT), and went tank spec, as it's the only paladin spec I care to play. Will I tank on him? If it is needed or wanted, and if one of the others who have a level 100 tank want to do something else, then I will. I haven't truly tanked since Blackwing Descent was current content back in Cataclysm (even then, only defeated the worm and robot council), so I am very rusty.
I thought of boosting my druid, but chose not to because I would have wasted the money boosting my int leather heirlooms to 100, so chose to boost something else. Leveling as boomkin, plan to hopefully heal some times.
Just play what you want. If you want to make another disc priest, go right ahead. There is no wrong choice. Except if you go Horde, as then you wouldn't be able to play with us. :)