Effects like card draw and lifegain can be extremely powerful, and some Limited and Constructed decks use blink or flicker effects to repeatedly trigger creatures' ETB abilities
Spend your first three turns foretelling and plotting as much as you can, before buffing 2099 up and having him blast your opponents or their creatures for huge damage
Hogaak, Arisen Necropolis earned its spot on the list for its brief notoriety following MH1s release, where it held Modern in a stranglehold
Matoya, Archon Elder is a sweet draw engine, and I can see her fitting into Kenessos sea creatures, Tales of Middle-earth blue-green decks like Elrond and Galadriel, and shell also just generally be a good engine piece in graveyard strategies that surveil a lot, like Mirko, Obsessive Theorist
If it is an item card, then that item can be played for free
Kefner's Last Word: With enough artifact mana rocks, the tap penalty can be minimized