Thunderlord's Decree is a very flawed Keystone for one very simple reason.
Auto-attacks count. Even non-empowered ones. I'd be cool with empowered auto-attacks counting (Talon Q, Tumble, etc). But only as 1.
My logic behind this? Auto-attacks are inherently sustained damage. Thunderlord's Decree exists to compliment burst.
Currently it's being picked on anything and everything to create burst. Even on the sustained damage dealers that I'm sure Riot always intended to have choose Warlord's Bloodlust, Fervor of Battle, or Deathfire Touch.