I just had a discussion with a guildie (a wonderful fellow resto druid) about this, who helped me really understand the mechanics here. I thought I might clear this up for everyone (although I'm pretty sure I was the only one confused in the first place. Bah).
Let's first look at the tooltip:
1. Heals up to 5 party OR raid members. So, pretty much anyone. We're all one big happy family.
2. 40 yard range. Just like a normal spell.
3. Within 15 yards of THE TARGET.
Wild Growth affects people who are 15 yards around the person whom you cast it on, not 15 yards from you. So it's not as though the spell has a range of 15 yards - you can use it up to 40 yards away. The range in which it will "spread" to five people total is 15 yards from your target.
Scenario 1: You have a group of people all around you, and one person 35 yards away from you. You put WG on that one person. It ticks on them and only them, even if that person is at full health, because there's no one else within 15 yards of them to have it tick on.
Scenario 2: You have a group of 5 people 35 yards from you, no one else is around. You put a WG on one of them. It ticks on all of them regardless of their health at that moment, because there are only 5 in the group.
Scenario 3: You have a group of 10 people all bunched up, 35 yards away. 4 are at 50% health, 6 are at full health. You put a WG on someone - it will hit the 4 people with 50% health, and randomly select one of the people with full health as the fifth.
Scenario 4: You have a full raid, 25 people, all closely grouped around you. Everyone has full health. You put a WG on someone. It will randomly select 5 people from the 25 (and not necessarily select your target) to WG.
Scenario 5: You have 2 tanks and 5 melee bunched up, 35 yards away. Both tanks have full health, but are about to take some damage. The melee all have 80% health. You put a WG on one of the tanks to pre-HoT him in anticipation of incoming damage. The tank will NOT receive the WG, and instead it will go to the 5 melee with 80% health.
I know I do.
To be honest, I wish it was a party heal. I understand that a smart heal, in most cases, is preferable. Consider this: Everyone is grouped up (think Thaddius). One person in each party takes 50% damage. WG on one of them and BAM all 5 are being healed. It makes sense, it's awesome. It's also moron-proof, and I hate that. It would honestly be more fun if one person in each group took 50% damage and you have to think "oh crap, I can't use WG, how do I go about healing everyone as efficiently as possible?" Instead of just hitting "4", you would have to *think*. Hey, back when I thought WG was a party heal, it was fun to judge how many people in a particular party had taken damage, and was it enough damage and/or enough people taking that damage for me to warrant a WG (Yes, you can laugh at me. Yes, I've been healing like this for the past like, 6 raids. BUT. HEY. I've been at or close to the top of most healing charts, so it's not - ok, ok fine. Keep laughing. I know. I deserve it).
The other problem is that you can never pre-hot a particular group. Prehotting (especially with Wild Growth) is awesome on, say, Maexxna (right before the web wrap), and Gluth (right before decimate). I mean, ok - you *can* prehot a group, but it's only going to hit that group if the group already has a lot of damage. If you want to prehot a group that is at full health, it will only work if they are in the exact position to guarantee that. Which rarely happens.
This makes WG (as my guildmate put it)a very REACTIONARY HoT. Every single other HoT druids have can be used preemptively, but WG cannot. Just something to keep in mind, and QQ about during Maexxna. ^^