User blog comment:PrayforDawn/What is Difficulty?/@comment-4846081-20140113002923/@comment-11581326-20140113030133

Riven's kit has no skillshots (Other than her ult second cast) and is basically either a q spam or q aa q aa q aa, making her not that difficult.

On the other hand, someone like Zed is VERY difficult to use properly because of the things you can do. With the recent change that makes his ultimate shadow have a global switch range, the possiblites of completely bewildering your enemies and allowing you to escape are huge. Add in flash, and his w, which can also be swapped with, and the places you can go are ridiculous. Thus, Zed has a difficulty of Nine.