Designers often focus so much on fairness and balance that they forget about the great entertainment value of doing something outrageous.
Most of your cards (or other objects) should be simple and straightforward. However, you should also include a few things in your game that break fundamental rules of your game, or seem too amazingly-good to be possible. When someone sees this thing, they should say ""WHAAA?!?!?!"" and immediately imagine the possibilities it opens up. Everything that doesn't just say "I win" should probably be considered for inclusion in your game somewhere.
These things should feel awesome but actually be entirely reasonable.
In Radlands, your location cards are the base for your game. They provide handy abilities that help your own people, or hurt the opponent's. The card "Reactor" blows up and kills all people in play. The card "Juggernaut" is a wheeled city that literally moves across the table, towards the opponent.
In my gangster game, locations let you roll various dice, which have effects on them. The "Corrupt Mayor" location lets you roll seven of the attacking die — a ridiculous number that's likely to eliminate all the other players. It's suitably expensive to do this, of course.
In Magic, there are "Planeswalker" cards. These are characters of exceptional power, who join you. Typically, each has three different abilities, and the last one is an "ultimate" ability. It might be the case that one of the other abilities is technically better value, but the ultimate ability is something big. Create five dragons, or remove almost all of the opponent's deck from the game, or gain 100 life (you start with 20.)
A good game is enjoyed while you play it. A great game creates an experience you'll remember, and a story you'll want to tell others.