have u ever used mental ray? if the answer is yes, then do i really need to explain this to you?
u'll find a lot of posts where people mentioned vray was much easier than mental ray and makes rendering fun again.
arnold is even easier than vray. it's pure brute force. no baking, no pre-pass, no cheating, no thing at all to help you reduce render time. u just set quality and that's it. minimal amount of settings you need to mess with = very easy. but at the expense of render time of course. GI is even on by default.
the nature of arnold enables traditional lighters to jump right in and place lights/reflection cards the way they would do in real life without having to learn anything much about the renderer.
shader wise, arnold is just about the same difficulty as mental ray / vray. tho i would say arnold's SSS shader is a tad easier to understand than vray once you get a hang of it.