I agree with Linden's post. I'm discipline specced while my buddy is shadow, and while he kills fast and seems to love it, It takes me 2-3 times longer to kill mobs my level and it's pretty boring.
There's one advantage to being discipline specced (or maybe it's just my play style) : mana. When soloing, I'm almost never under 50%, and I by the time a fight is over, odds are I have the same mana as when I started. I basically never have to sit down for a dirnk unless I got aggroed by multiple mobs. When grouped with him, after we kill 5-6 mobs, he's out of mana while I'm still full.
Priests is actually pretty similar to Warlock in a way. I keep my mobs DOT'ed throwing in a nuke once in a while, and while I don't have a pet, I melee and use my wands to deal the rest of the damage.
At 38, if I fight a mob 38-40, I basically cast 5 spells in a fight : pull with a Mind Blast followed by DOT then I shield myself. My shield will usually last till the DOT is done so I refresh both and keep attacking with melee or wand and by the time the DOT has faded, mob has about 100 hp left. Mind you, I'll use Inner Fire when it's needed and every once in a while I'll also need a renew, but these don't cost too much mana.
To get back to the initial question : A priest is very easily played and if you don't get adds while soloing, you never die. The downside is : soloing is slow and gets boring fast if you're not shadow specced. In a group, it gets a bit more complex, but it's a lot more fun then soloing and after you get the used to a particular group's strategy, it's also fairly simple.