backgrounds sound like your "Interaction Class", to me. Some interaction may get smeared into you Class, but I expect Background to do the heavy lifting. I'm not sure how to figure where Exploration will be carried. Obviously some must come from class, but will Theme or Background carry the rest? Combat, as we've pretty much been told, will be shared between Class and Theme.
I'm not even sure balance applies to Backgrounds, though. I mean, in some campaigns, the "merchant" background with all its extra languages (or whatever) will be totally irrelevant, but in others, he will be king. In the same way, an Aristocrat's trait to demand hospitality from other nobles will be useless in a campaign taking place on the untamed fringe of the known world. Such things could even vary between adventures in a longer campaign. So my guess is, that rather than try to make sure all the Backgrounds are balanced, that they will try to make sure that none of them bring any "gamebreaking" traits into play.