I commented before how dreadful priests were at the early levels and that was by no means a joke. Not only are priests pre-level 20 dreadfully boring but they are also dreadfully mana inefficient. I've played every single other class before and I've never experienced the boredom this class affords anywhere else. Early on your only offensive means are Mindblast, Shadow Word: Pain, your wand, and Smite if you actually decide to use that. You'll usually open up with Mindblast, then Shadow Word: Pain but you'll be bubbling and wanding from there. I found Smite to be of little use and actually do more damage with my wand, plus my wand doesn't cost me mana like Smite would.
I'm wearing the Devout Aurastone Hammer with +20 spirit, Tattered Dreadmist Robe and Shoulders, and two Discerning Eye of the Beast trinkets so I'm probably a little better off than someone who is just fresh to the game and is playing a priest. I made the mistake of not putting two points into improved spirit-tap initially and I'd recommend you not follow me down that road. With it being tied to crits and me not having a whole lot of that I thought it would be virtually useless. Point in fact it doesn't proc as often as I would like because of my low crit rate but it's also the only bit of additional in-combat mana regen I have until l can start putting points into Focused Mind. My advice to all new Shadow Priests is to put your first five points into Spirt Tap and Improved Spirit Tap. But you won't get Spirit Tap procs unless you get the killing blow. Makes instance runs a real pain, especially pre-level 20 when you'll be spending twice as much time wanding as you will be casting anything. By level 30 it's much better as you'll Mind Blast, Holy Fire, and Mind Flay and will get a good share of killing blows. Improved Spirit Tap comes in very handy as even with 6% crit I see it go off regularly enough in fights for it to matter.
I'm starting to enjoy the class more now that I have some additional tools and I'm starting to get into the talents that increase my offensive abilities as well as decrease their cost. If I had a way to skip over the 40's and 50's and go strait to Outland I would because I know from level 60 is really where this class begins to shine.