The real issue with the Druid mastery is how poorly it compares to the Shaman mastery and how they are implemented. The Shaman mastery is simple: Health is low, heals hit harder. It happens 100% of the time, and the extra healing % applies to the entire healing done for the mana spent on that cast. There are two ways to implement the Druid mastery, though, and neither are ideal. If the HoT's recalculate at every tick, which seems intuitive, then you will only get a portion of your mastery bonus for the mana spent on the HoT. If it doesn't recalculate, but rather gets applied at cast time, it is even worse, because the bonus is to HoT's at a time when you really want a direct heal.

In a raid setting, they are not much better. In the first situation, the shaman will still get 100% of his mastery bonus and will likely snipe your mastery bonus when his boosted heal takes the target out of the danger zone in the time your boosted Hot gets to tick once. In the second situation, you will either be ignoring HoT's in favor of OMG HEAL HIM HE'S GONNA DIE or it will end up forcing Druids back into a Wrath paradigm of rolling Rejuv's just to maximize the benefit of your mastery.

TL;DR Druid mastery doesn't play nice with the rest of the raid and is counterintuitive to what a healer wants for his group. it needs to be changed.