From a design (and not balance) perspective, I think that SOME keystones have failed
In game, players are encouraged to make themselves unique through the keystone masteries they take. This worked definitely on a few keystones like Thunderlord's decree, which has a huge explosion every once in a while, but not on others like Warlord's Bloodlust.
Why? Because they're flat stats. Flat stats are the bane of identity. I honestly think, that Riot should put someone like CertainlyT to specifically make 9 unique masteries. The entire goal of masteries begs for it, while people have complained that some champs were broken compared to others as we all know CT does tend to sacrifice balance (and occasionally counterplay 


amirite jk jk don't hunt me down yasuo mains) for completely unique passives and abilities, masteries are accessible by everyone.
Things that make a mastery unique: Not easily accessible, Visual effects
Failed
- Strength of Ages - Really just a limited Sion passive, flat HP per kill...
- Fervor of battle - gain more damage on auto attacks if you use it more - flat stats,
- Warlord's Bloodlust - a flat stat reliant condition (crit) that gants another flat stat (lifesteal),
Debatable
- Windspeaker's Blessing - this one is really debatable, if intelligence (+5% CDR and its cap) was a keystone, it would be like Windspeaker's Blessing, it breaks the rules for flat stats. I really think it could be better but it's not the best example. It lacks a visual effect.
- Stormraider's Surge - I really want to put this on succeeded but I can see a few arguments, but is seriously borders succeeded undeniably. On one hand it's just speed, on the other it has a cool visual effect and speeding up is noticeable.
- Bond of Stone - Also very debatable. I think there's no visual effect, but extremely unique so it's also borders succeeded.
Succeeded
- Grasp of the Undying - I may have never noticed it before but people say it has a visual effect and it's unique so that's pretty great.
- Deathfire Touch - your enemies set on fire, that's pretty great, visual effect and all. However, it should be different, if not opposite visually from Corrupting Potion. Make it extremely noticable, like fire coming to life in a quick period of time, a blaze or explosion.
- Thunderlord's Decree - Huge explosion, sudden damage, visual effect, sometimes I have trouble seeing the AoE clearly though and the AoE is kinda small but that's another discussion in itself.
When I heard about keystones (and they weren't released or revealed yet), I thought "Wow so there'll be more glowy stuff, projectiles and explosions in a game? COOL!" but it kinda didn't work out that way.
Maybe Windspeaker's should've had a sphere around you to represent more resistances, or even made the healing even more but made it a decaying shield. Maybe Bond of Stone should've looked like Kalista or Illaoi's visual effects with Oathsworn or pulled spirits. Maybe Warlord's bloodlust should've looked like Soraka's self healing from hitting Q and scaled with AP / HP. They should be as noticeable as Ashe shooting a volley not looking like Varus shooting his arrows.
I think they should've made Runaan's look like a mastery.
EDIT: Yay downvotes for trying to make a meaningful discussion!
DOUBLE EDIT: OMG the bandwagon voting has been defied, HOW?!