Support is the hardest role of all. You have to know when to ward, where to ward, they control the flow of bottom lane so when to get aggresive etc. Ad carries just sit and farm.
I've said this before, but it's a different skill set. Support is easy for me because I have good knowledge of the game, but ad carry and most tops and mids are hard for me because my last hitting wavers between slightly below and slightly above average.
Not taking into account mechanics, I still reckon support is harder because you have to consider so much more. It might not be mechanically challenging, but it takes a lot of game knowledge to be a really good support.
dont read if you havnt seen the game yet
Best game I have ever witnessed in LoL history, and I saw it all live. Watch the entire thing if you haven't heard what happened already.
http://www.twitch.tv/dreamhacklol/b/321786472?t=150m
Here it is on youtube if your computer can't run twitch without lagging.