From the Druid Preview:
Eclipse: We are moving Eclipse from a talent into a core mechanic of the class and making it less random. Balance druids will have a new UI element that shows a sun and a moon. Whenever they cast an Arcane spell, it will move the UI closer to the sun, and buff their Nature damage. Whenever they cast a Nature spell, it will move the UI closer to the moon, and buff their Arcane damage. The gameplay intention is to alternate Arcane and Nature spells (largely Starfire and Wrath) to maintain the balance.
Systems like this sound good at first, but they rarely work as intended. To see why, think of the proposed UI element as a state-machine, instead of a bar. You end up with a "chain" of linked states. Each state has two possible actions: an Arcane spell or a Nature spell. Casting an Arcane spell moves to the state on the right, and casting a Nature spell will move to the state on the left.
The key here is that for any given state, one of the two possible actions is better. In State 1, the best move will be an Arcane spell. In State 4, the best move will be a Nature spell. This is true no matter how you get to the state.
You will eventually end up oscillating between two states, regardless of how many possible states are in the chain. In State N, the best option is Arcane, which sends you to State N+1. In State N+1, the best option is Nature, which sends you back to State N. And then you bounce back and forth between those two states forever, regardless of how many possible states there are in the chain.
If you don't want this to be true, you need a third element--some *extra* buff or debuff--which in State N sometimes makes Arcane the best, and sometimes makes Nature the best. The state chain itself, and any bonuses or penalties inherent in the individual states, is not enough.
Of course, you can mitigate this a little. Maybe the new Nature Torrent spell jars you out of the oscillation and sends to a state much further up or down the chain. But then you'll still end up oscillating around a different point, until you get to Nature's Torrent again. I don't think this is really what Blizzard wants Balance druids to play like.
(I could be wrong, however. Maybe they do want Balance druids to go Wrath-Starfire-Wrath-Starfire-etc. The last line of the preview is inconclusive.)
What I would do is relatively simple: All spells move the UI element in the same direction. At first, the bar would move towards Midnight. After Midnight, the bar would move towards Noon, then back to Midnight, and so on. During the Night, Arcane would get a bonus, with the highest bonus coming at Midnight. During the Day, Nature would get the bonus, with the highest bonus at Noon.
Of course, this makes Balance theorycraft pretty simple. Cast Wrath during the Day, and cast Starfire at Night. Maybe refresh Moonfire at Midnight and Insect Swarm at Noon. But simple might be best, if it still results in something more than a two-state oscillation, and is also easy for everyone to understand.