They are calling this the rarity key, and the rare, super rare and legendary symbols will be holographic on the cards (or some kind of shiny gradient), to make it even easier to differentiate
Part of the reason targeted discard is so popular in Modern is that Thoughtseize is basically the Force of Will of the format, being the best option to make sure you don't die on Turn 3 to a deck like Storm or to a Through the Breached Emrakul, the Aeons Torn
Every pack includes at least one Traditional Foil, with 1-4 cards of rarity Rare or higher
Infinite ETB triggers Karmic Guide + Mirror Entity + Reveillark + Creature with ETB trigger (Acidic Slime or Derevi, Empyrial Tactician works especially well
Twelve loyalty is so much, even with all your planeswalkers proliferating, and picking them off to take the extra turns weakens your board since your superfriends deck ostensibly wants those extra turns to activate its planeswalkers again
His activated ability allows you to tap artifacts for blue mana, which not only accelerates your mana but also synergizes perfectly with the numerous artifacts in the deck