First DK are faceroll (from a dk)
Second paladin are actually far more complicated than most classes now. Some people are still stuck in the wrath of the idiots expansion, but that time has come and passed.
Paladins are one of the most complex classes to play, and I say that because having played one you must be aware of many procs/cooldowns to do well.
Procs:
their mastery (I forget the name) is a proc with a chance of making your next ability that consumes hp work as if it had full hp.
divine purpose certain abilities have a chance to generate holy power
art of war makes your attacks have a chance to proc for free instant exorcism.
Now that's only 3 procs, most classes only have to worry about 1.
Even then it gets more complicated.
First a portion of their dps is dependent on a buff called inquisition. The inquisition boosts holy damage, but it consumes holy power when used and only last what 8 to 20 seconds. Letting it drop off can be a real problem, so you have to plan in advance for when it will drop off and conserve enough hp to use it. Then there is zealotry that requires at least 3 holy power to use, but doesn't consume them. It makes it so your crusader strike generates 3 holy power.
Now complicating it a bit further is the use of mana for some abilities, and HP for others.
I won't say that Ret/prot is the most complicated of specs. I will say they are a damn sight more complicated than things like mage (my alt), druid, dk etc. There are only a few classes that aren't faceroll right now.
Paladin, priest(disc/holy), shaman, and lock. The rest are so easy it's boring. How do I know? My three mains are DK (lolfaceroll), Mage (again lolfaceroll), druid (healing on him is not faceroll, and he can burn mana quick if I'm not careful)