In fact, except for a few bosses that need to be moved intelligently, tanking always was easiest.
Healing also was easy since BC. You just mash your buttons. Mana is irrelevant. Every GCD you change the targets of your heals with your mouse on grid. Paladins don't even do the latter. It is rediculously easy.
Damage Dealers, on the other hand, need to do max dps and can be compared very easily. Max dps often means following a complex rotation or 'If-Then' rules. If they don't do enough dps they get sorted out. If the tank has threat problems they need to be careful, while still doing max dps.
Damage dealers need to look out for procs, available cooldowns and cooldown stacking. Some of them even need to care about mana. They need to target the correct add and if they do pull aggro they need to use escape mechanisms to prevent dying. They need to interupt spells, slow enemies, manage ressources.
Now, the reason I post this is that Ghostcrawler just partly agreed with me.
I'm just reading a lot of "tanking is so hard" threads, and A) we don't really agree, and B) if it's any less challenging I fear a lot of tanks may get bored and move on to other classes. There are some annoying parts of tanking we need to fix, and we definitely want to fix the threat scaling problem and any major disparities in AE tanking. But if you're looking for an experience even easier than tanking Naxx, I'm not quite sure how to provide that given my A and B above.