Unlike other healers Druid has unique interaction with secondary stats and his abilities.
While other healers by getting haste only increase their cast time of spells, meaning they shoot them out faster, meaning that their Throughput increase equals mana consumption increase.
So for example a Priest with 30% haste will have 30% more HPS but a cost of 30% MORE mana consumption. Same goes with all other healers except Druid.
With 30% haste, Druid has 30% increased HPS with no increase in mana consumption, as all of his HOTS last same, base duration, while they get 30% more ticks of that HOT. Meaning that as the game progresses and higher ILVL items with higher stats are available, Druids are the ones that will have highest HP per Mana healing in the game.
Having their spells be so versatile as their kit, there simply isn’t a bad stat for Resto Druid.
even Critical is very usefull as Druid’s healing is split in TONS of little ticks over time, so it becomes a linear increase as each little tick , crits separately for itself, rather than spell as a whole.
When you would put all healing specializations one next to another, and they all had 20% haste, Druid would have 20% less mana cost compared to them. Which I think is HUGE.
Especially that everyone will have at least 15-20% haste to make their class more fluid. So after building Mastery to say… 30%, everyone else is starting to fall behind on resto Druids, Not only because Druids will have 20-30% less mana cost compared to them,
But also because Critical for example is very unstable stat for other healers, Priest, Shaman, Monk, Pala.
You cannot wait someone to loose 40% hp before you toss a heal their way, because otherwise you might CRIT and overheal them. So Critical becomes a stat that looses some of its percentual increase in random overhealing. But not for Druids
Versatility itself is very expensive stat to build, so if other classes have it as best secondary stat after Mastery , than they are already slowly falling behind the rest…
TLDR: As expansions progress and ILVL rises Resto Druid will become unstoppable monster compared to other healers.