They all tend to play with the graveyard or your life total in some way, encouraging sacrifice, self-mill, and recursion to a big degree
For two blue mana, Flesh Duplicate provides you with the shapeshifting answer to a surprisingly large number of threats
Currently, every main MTG set releases with: Play boosters - packs of 14 random cards including at least one Rare or Mythic Rare
Setting up entire combos around creatures leaving and re-entering might sound a bit pointless until you remember the myriad enter the battlefield (ETB) triggers found in a huge number of creatures
This usually wont come up, but its entirely possible that your opponent might have an Ornithopter of Paradise in play
You cant focus on card advantage and have a good way to use your extra mana early in the game, so the opponent wont be able to keep up