The problem is it is just very inconsistent. You can pull off amazing combos but are reliant on drawing the cards at the right time. My priest deck is one I have invested the most in, both time and dust wise. I am not saying it is impossible to rank up with a priest, it is however the slowest of all the classes by far.
I fluctuate between ranks 10 - 13 and sadly a typical session of ranked looks like this:
Play priest deck : win 1 out of 3 games on average
Switch to aggro warrior at one third of the dust cost: win 90% of games, get win streaks and hit rank 10.
Switch back to priest: again slowly slide down the ranks.
Even some of the best Priest streamers such as Era or Amaz are switching classes. Era rarely plays priest anymore and Amaz plays other decks off stream to regain the ranks he loses while playing his priest deck :( WTB a new class card that will make us less reliant on very specific combos to pull of wins. I would love to instead of closer games more consistently rather than either me absolutely destroying the opponent or getting destroyed.
I have tried all sorts of variations of priest decks, auchenai/circle of healing combos, wild pyromancers for anti aggro, divine spirit/inner fire cheese, mix of all of those as well as deck lists of players much better than me. Just can't get to a comfortable win percentage and I guess I get jealous of the 85-90% win rate of my warrior deck =/