I loved season 4 and some of season 5.
The problem with league currently is that the most optimal playstyle has nothing in common with normal gameplay.
AD assassin mid=> if the enemy team is smart, they will lock 3-4 tank/juggernaut champs and make the guy ragequit. There is a reason why AD assassins are generally not played in competitive. Noone wants to deal with 1 guy 1 shotting left and right. It doesn't matter how good the X player is, if you can counterpick and counterbuild. So, we have overpowered champions in soloQ that won't be picked in e-sports despite them being good, because they can be countered.
ADCs are overpowered from season 1 and the metagame/general strategy and tactics of the game are pretty stale compared to other mobas (still individual mechanical skill for playing a lol character takes more than playing a generic character from other moba, but there is nothing exciting to do on summoners rift for the old players - the game was already solved by season 2).
The whole runes/masteries system is beyond broken and balance team made some really bad mistakes in the recent years - ADC/mage/assassin patches. Only the juggernaut reworks were good after some balancing.