A lot of people believe that keystones shifted the game out of balance, with some champs being able to abuse them better than others. They also added power to champs that was not calculated for when the champ was designed. Colossus on Vi and poppy is an example. Warlords on a fed adc makes them more survivable than riot ever planned for when they created the champs.
It's also worth nothing that keystones ruin the game entirely around level 18, where half the players have them and half don't.
Was this game better without the shifted balance from keystone masteries? Vote below.