Can anyone explain how much spell haste will affect druids in patch 2.4, whether positively or negatively?

positively, as it will reduce the GCD. For trees, it will be a stepwise improvement going in ticks of 126 haste (iirc), as it will increase the number of spellcasts you can get in before having to refresh lifebloom by one. As for moonkins, from what i've seen with my gear and spreadsheet, 1 haste ~ 1 crit rating in terms of increase in DPS.