• How much work is it? How many new models, sounds, effects, etc?
• Does it deepen the Heavy's skill curve? Is it easy to learn? Hard to master?
• Is it an interesting tool to choose relative to the base Heavy weapon it's replacing? What scenarios can you envision in which each is useful? What arguments can you raise for why each is better than the other?
• How often does the Heavy need to think about it? Is it something he uses once every 5 minutes, or is it something he needs to be constantly thinking about? A greater impact on player decision making is generally a good thing.
• How many other features of the game does it affect? Often, the best ideas are "economical", with a small set of required actions, but a wide set of resulting effects.

Via Team Fortress 2 Blog