While I don't think it is necessary for the 15% nerf, the argument can be made.
Warlocks have additional health due to their method of getting mana back (Which is converted from their life). Their damage CDs are however pretty ridiculous (Dark bargain, Unending Resolve, Soul Link, etc) in not only the sheer amount they have, but the versatility they provide.
Moonkins are quite different in how we use CDs, as well as what our CDs provide. Symbiosis allows us too have our major defensive CD change based on fight (AMS for ionization on Jin'Rokh, Cloak for Static Shock on Lei Shen, Unending Resolve being our good cookie cutter defensive CD), as well as Barkskin (While being fairly weak, does have quite a low CD i.e. can be used on every Quills, Lightning Storm, etc). Heart of the Wild also provides us with 6% additional Stamina, which is severely underestimated or ignored by many (Which will also scale with higher stamina levels for next tier).
The Nature's Swiftness change is something that I don't quite understand, and while we do have other options, the lack of instant cast BR is more annoying to me than taking renewal or the new talent.
All in all, the 15% defensive change hurt as bad as many think it will when compared to other casters, as S priests are losing theirs and Warlocks are losing their 10% reduction as well. Hopefully blizzard comes through with trying to make another form of active mitigation for Moonkin too put us in line with Warlocks for survivability.