A comment on my prioritisation post made me think that perhaps this was a worthwhile topic for a full post :)
Lets start with Tranquility, this is clearly the spell that is often forgotten. I think that many druid are a little reluctant to hit that button, it's such a great spell and it does have that 10 minute cooldown.
Firstly 10 minutes really isn't that long, if you progressing through Ulduar this basically means you can use it every 2 attempts (not taking into consideration trash). I guess a simple way to look at this is if you go more than 2 bosses without using it then your not getting much out of it :)
Secondly I personally use tranq as a catch up spell, it is a great spell, can be a little mana hungry but is a great way to catch up on heals if your raid is taking a lot of damage or you happen to be down a healer or two.
Wild Growth on the otherhand has quickly made its way into the basic play style of many resto druids.
Fact is that Wild Growth is considered a "smart spell", meaning it should target those who are in the greatest need of your heals. Even post nerf (intro of our cooldown), it is a powerful raid heal which has the potential to take up a large percentage of your heal stats.
Their are still some druids out their not using the spell to its full potential and I recommend you have a play with it.
Also it should really be even more powerful in 10man content as your healing half the raid!