Mistrise Village is the perfect example, giving you protection for your key plays on what is pretty much always an untapped blue source
Ilharg, the Raze-Boar gives this deck an extra line of attackthe deck can reanimate the Boar God and use its Through the Breach-like ability to put something like Griselbrand or Emrakul, the Aeons Torn into play from hand, to either kill the opponent in just one attack (as early as Turn 1 or 2), since an attack from Ilharg, the Raze-Boar plus Emrakul, the Aeons Torn amounts to more than 20 damage, or to draw enough cards to combo off again on the next turn with Griselbrand
Honorable Mentions Amethyst Chromicon has the drawback of aiding your opponent but adds a lot of consistency
This card enters the battlefield tapped and can be tapped to add one red mana
No Symbol holos are a recognized and actively collected variant, and they are Unlimited cards
In a deck with a five-color commander, it essentially reads, "You win the game if you control your commander." That is not the most satisfying end to a game, and it does begin to become an auto-include for many decks