First of all, the Enchanted creature gets +1/+1 for each +1/+1 counter on other creatures you control. This is for each counter, not just each creature. Plus, it gives the creature vigilance and the ability to pay 1 White mana and tap to Bolster 1. Because it's an instant speed ability, it can be used at the end of an opponent's turn.
The best use for this card, sadly, would be in a Heroic deck, which has all of its parts rotating when Theros block leaves Standard in October 2015. That being said, there are creatures that can amass +1/+1 counters, such as Servant of the Scale, Anafenza, Kin Tree Spirit, Avatar of the Resolute, and Managorger Hydra, among others.
But wow this card would be awesome in a Heroic deck with all of the +1/+1 counters that deck produces, and Hero of Iroas making this cost only a single White mana. Plus it even gives you another way to Bolster. This card just seems too good to not ever see play. It may still find a way. There are enough +1/+1 counters running around for this to do something mean.