Every single class is easy to get good at, some are harder to get optimal at. As far as pve dps goes feral druids are probably the class/spec with the highest skill cap at the moment, rogues and warlocks are generally regarded as easy classes to master. For healing I'd say monks might be the hardest but beyond the endless over healing and number inflation healers just end up doing it doesn't give much insight, the little I've healed as pally in MoP you basically spend your holy power on sending out hots on the raid when you got time to do so and heal tanks when needed + aoe heals for aoe damage phases, easy. Tanks are about equal I'd say, they all have some bumps to their arsenal but as long as you can keep aggro and keep a somewhat stable rotation for your defensive attacks and defensive cooldowns you should be more than fine.
I have no idea about because I gave that up after wotlk but I do the odd bg from time to time and casters/ranged have way, way, way too many abilities and tricks to get away from melee. Only way I can kill people as a ret pala is if I pop every single fucking cd I have and kill them fast during a stun. If they live then they just run away or start kiting.