With all due respect to the brilliantly skilled priests out there (because, seriously, you're amazing), I've never been able to get a priest all the way. Near max level, some PvP, and even a bit of grouping, but I've never gotten a priest to max level and kitted up for raiding. While not ironic, I still note that it's the only class suffering that malaise for me. I've stepped foot in raids with every other class, and done some decent progression among most of them.
I'm not sure what the hangup is. Maybe it's because I think of the priest as being my wife's class. In the same way she hasn't mastered my main (Paladin), I just tend to feel like I'm treading on her territory when slinging them priest spells.
What about you? What class hasn't really jived with your playstyle, and why not? After nearly a decade of game, you'd think they'd all be old hat by now. So what's your missing piece and why?