of course there are class/specs at 39 that are *more potent* than others.
more potent in certain situations, yes, but most classes excel if simply played well as long as they spec their toon properly. no class really stands out unless their bane(s) isn't present.
warrior: high burst potential, can pull off chain crits but are easily CC'd once intercept is down
rogue: great at CC, stunlocking is very effective but their overall effectiveness decreases as group sizes increase. flares in chokepoints are a bitch.
hunter: MS effect, slows very well, high DPS and good burst potential, easily CC'd though
shaman: very versatile, can do a bunch of stuff but nothing too effectively. burst is within windfury, CC within FS/EB/grounding/wind shear, survivability with GW/heals/stoneclaw, sort of like a jack of all trades
druid: best burst healer, can track/fairy fire/root/abolish for utility, but stuns and mana efficiency are a big counter
paladin: best mana efficiency, JoJ is very useful, and very useful CDs. heals are disrupted easily and burst has been downsized though
lock: when the dots are not dispelled, epic DPS. when they are dispelled, mediocre. great survivability if played right.
priest: dispel is very useful. heals get better as groups get larger. not good against burst and mana can be an issue.
mage: versatile, good DPS if uninterrupted, high burst potential, cloth armor + SP gear = quick death though
i dunno that's a general outline, kind of rough but i hope it helps the decision