The challenges of game balance

I think Understanding Balance in Video Games is a good article that points out some of the things that game developers need to think about in regards to game balance.  What I like about articles like this one is that it may not explain how to achieve game balance, but it at least gets you thinking about the issues.  I had not thought about some of the finer details of game balance and I also think I am beginning to understand where complexity comes from in a game.  Take Radioactive for instance, the game mechanics are fairly simple because the physics engine does most of the work, but the levels can be fairly difficult to balance properly - how many objects do you include within a level, what types of objects do you have, and what is the physical layout of the level?  When you consider that every collision surface within the level is potentially part of the solution, it can be difficult to add the right amount of challenge so that the level is not too easy and not too hard.