I managed to make it to level 70 as a Shadow Priest. My first comment is that I still feel that Shadow Priests are the worst class to level. The fact that you’re single targeting and lacking any kind of cleave and/or AoE ability until level 74 (I discount Holy Nova btw) makes instancing a real pain in the ass. I’ve noticed that outside of boss fights, my priest’s DPS is sorely lacking. I know for a fact that shouldn’t always be the case as I’ve seen quite a few Shadow Priest in LFR hit the top of the damage charts. But at the moment, it’s been an endearing effort.
The play style is quite similar to a warlock except you really don’t get a permanent pet. However, the dots are simpler in that you mostly worry about two main ones. The one thing that does bug me is that long casting time for vampiric touch and mind blast. Quite often I’m finding that by the time I’m about to cast mind blast, a mob will die too fast. So I end up utilizing far too much mana using the raid rotation.
I think the only aspect that might be interesting as a play style is the AoE healing that a priest does during single target mobs. But in terms of leveling, this aspect does not feel very useful. I think with Cataclysm and all the party-wide damage that occurs, the AoE healing makes shadow priests more interesting to play. But during the leveling process, you really miss out.
Finally, there’s a lack of movement that you don’t get until higher levels. Probably, the only benefit is levitate but even mages get that. If anything, you’ll get perhaps a single point to enhance your movement at best.
Overall, I find the shadow priest a very frustrating and non-rewarding class to play of all the toons I’ve leveled so far. I’m still going to continue pushing until I can hit 85 just to compare the play style. But it’s been a real pain and certainly not at the top of my list to hit level 90 when Mists of Pandaria is released.