League of Legends big nerf explained
The big point that Lead Content Designer Morello makes, is that "power creep," is out of hand. This is when new items and champions make older ones feel redundant and in efforts to make unique new content, Riot thinks it has neglected early arrivals to the league roster.
"Power creep is very insidious over time, because it happens slowly and gradually," says Morello. "This leads to a problem to where power gets in the way of decision-making and counter-play by creating an arms race between competing factors (champions, in many cases)."
This is something Riot wanted to avoid. While it likes the countering system in draft pick games, it didn't like there being only one choice to beat certain overpowered champions. Apparently this happened a lot in Season 2. In Season 3 it changed items to help and nerfing certain champions was the next step.