RPG are games, and games are meant to be fun, so you could say that fun is what game design should lead to.
Unfortunately, fun is relative: I might consider something fun, while for another person the same activity could be extremely dull and boring. For this reason, you basically cannot design a funny game, just a game which produces certain activities (usually, activities that you consider to be fun).
"Balance" is a terribly vague word, that can be related to fun in various ways. For example, someone may say that Rolemaster is horribly unbalanced, because the player impersonating the GM has an absolute advantage over every other player, but that specific lack of "balance" is part of what makes RM fun.
Or, in Fiasco there's no GM, and every player has exactly the same chances of influencing the outcome of the story. It's a really fun game, but it gives you a completely different experience from RM.
Or, d&d 4e is considered to be extremely balanced because a party of PCs has good chances of winning an encounter of their same level, using a predictable amount of resources. This may be fun if you like this kind of tactical resources management, or boring if you don't.
So I think that "balance" is important only in relation to the activities your game was meant to produce. In other words, it's not really important how a game is balanced, but why it's balanced in that way.
Finally, realism: I wouldn't put it as a generic point of game design. I mean, if I'm playing Gamma World, or Toon, or Primetimes Adventures, the least thing I want is realism.
I'd exchange realism with "coherence", in the sense of "producing the activities the game was meant to". Like, if I'm playing Toon, I want to tell silly stories about cartoon-like characters. If the game gives me that, then everything is good.