As of October 2012 League of Legends has around 32 million active players.
Less than 0.5% of all players on League of Legends actively play the Dominion gamemode. This in turn led to the eventual removal of this gamemode entirely, until it's return in the game's rotating gamemode queue.
During the development of Jhin, another champion that was released before he was, Kindred, stemmed off an initial concept for him. Kindred's bounty hunter-style passive was originally going to belong to Jhin, following the bounty hunter sniper theme that was originally planned for him. Once the concept was scrapped however, this passive ability went to the personification of Death that was, Kindred.
Riot Games has used Urf the Manatee as an April Fool's Day joke. The first year it was announced as a Champion for the Beta Test as well as the following year.
The Warring Kingdom skins worn by Xin Zhao and Jarvan IV are a reference to Koei's game Dynasty Warriors based on the Romance of the three kingdoms. Xin Zhao as Zhao Yun and Jarvan IV as Lu Bu.