The Herald creates a brand new level of tempo for Enchantment based decks. Black/Green Constellation has been a decently competitive deck on Magic Online. during Theros block. Making what are already fairly costed cards even cheaper, plus the bonus lifegain, makes the deck more formidable. Granted, this upgraded Constellation deck is not going to have a long Standard lifespan. But depending on what Enchantments are printed in upcoming sets, the Herald could be an important part of some sort of deck.
A 2/2 for 2 is a bit fragile for Modern, where Lightning Bolt can easily pick it off. But in Commander, where there are plenty of Aura and Enchantment based decks, the Herald should have plenty of fans. Some Modern Enchantress brews may pop up with the Herald now in existence, but its fragility could make that archetype difficult. In any case, this creature provides plenty of value.