They spent every season tweaking the game more and more; altering mastery trees and re-balancing with a particular goal in mind. No season felt especially different from the next. What I mean by that is, adc's generally played the same way. Assassins generally played the same way. If you were a riven master one season, you were likely a riven master every season. Etc. The core gameplay never really changed even if certain champs or roles were buffed/nerfed, or mastery trees were altered.
Then came pre-season 6. Roles don't play the same at all. Everything that was so tweaked before was turned on its head. Towers went from being a strategic element to being a side-walk cone that you kick down during your sprint to the enemy nexus. Champs that were powerful if mastered, yet difficult to master, like Yasuo, became so easy to play that noobs started getting pentakills with him and playing him in every game. Champs like Riven became trash and unusable. One of the more unique roles to play, adc, became easy to play, overpowered, and could go anywhere, making other champs completely useless in those lanes.
I'm not sure if the programmers played too much HoTS and thought that was a better game and tried to completely revamp LoL, or if they were trying to make it more urf-like, or if it's some grand plan with the way they see Pro games going, but I agree, LoL has lost its magic somehow. It's not the same game in its current state. I feel like they took away a lot of its more intricate qualities for cheese factor.