i leveled my priest up with a rogue and a mage. between the three of us we dueled constantly. vs the mage we where pretty rounded off, but against the rogue i got smashed until i hit 40 and picked up my shadow form. Its hard at lower lvls to win as a priest because you dont have the armour or the health to withstand the often times brutal tactics of the other classes at your lvl. Once you hit shadow form though, you get that 15% damage reduction, and trust me, you notice it. also your spells pick us more damage so they will be crying to their mothers every time you start your DPS. As far as your crits, dont really worry about them. While yes, its nice to be able to say, oh wow i critted for X amount. Dont expect it. You are a priest, your attack spells will not be as strong as many of the other classes. Use your mind flay and your Dots for your primary damage. they are constant source of DPS and once you get rolling, its hard for someone other then yourself to stop you. Mind Blast is your nuke. Use it as such, if you have an openning, blast them, its funny to see them say "OH GOD" because you do a steady stream of damage and then nuke them. also, like it was mentioned earlier. Heal yourself and keep up that shield as much as possible. in your bracket that is pretty much the only way you will stay alive, well that and fear, but its kinda worthless with all the nerfs.
Me personally, my hardest fights are still vs. rogues(though CoS is funny to me, most rogues abuse it and i just take advantage of that) and lock if they get the jump on me. Just about any other class, if you learn how to fight them, you will win 3 out of 4 times.