Good ideas have a few distinguishing characteristics:
1. They are simple. Complex ideas fit much more specific niches, are more difficult to balance, and may not combo with existing cards as well. Someone might think your "Nuclear Tiger" creature card idea is cool because it can use a rocket pack to become airborne, can destroy permanents whenever its health is below half, and its attack is dependent on the number of gravity quanta the player has... but that's not a good idea because it's too complex. Stick to one mechanic and see if you can put it in an interesting framework.
2. They revolve around a mechanic not yet seen in the game. Thus a permanent that boosts Fire creatures' stats while on the field is a rehash of Nightfall and is not a good idea; an Earth creature that gets 0|+X, where X is the player's shield's damage reduction, is a good idea because we don't have any form of that in the game right now. This isn't totally black and white; some good card ideas may revolve around putting an ingame mechanic into a new framework - like taking a spell card that already exists and putting it "on a stick" in an interesting way by making a new permanent that can do the same thing in some limited fashion.
3. They are reasonably balanced for the game as it exists at the time of posting. You may not foresee all the combos your card could produce, but that's one of the hallmarks of a good card idea: flexibility. This forum is an excellent tool for ferreting out the possible abuses you didn't foresee. Input is important here; points 1 and 2 are entirely your job, but the community helps greatly with point 3. Listen to the community, and address concerns in a reasonable manner without becoming defensive. Balancing your card idea makes it more likely for that idea to advance to the Forge and Armory, and also more likely for Zanz to give it a second look.
