This can of worms has been open for decades now: divine magic vs. arcane magic vs. psionic power. Transparency was a serious technical issue, but power-sources for magic are even more of a hot button because they challenge the concept of ownership of power. Some people are really averse to the idea that they don't own their magic. If a patron deity frowns on your behavior you lose your cleric spells. If you double-cross your arch-fey patron suddenly you're not leveling in warlock anymore. You broke your Code of Conduct and now you're a Fighter who is missing a ton of feats.
Part of the power-mongering twink-appeal of the Wizard, Sorcerer, and the Psion is that you don't owe anyone anything. It's your power. If you want to be a chaotic "neutral" wanker and murder townspeople, kings, and deities nobody can take your super-powers away. You're just better and all the other players and NPCs can suck it. That mentality is part of the reason why people started freaking out at the idea of the Next sorcerer being transformed in form and personality - loss of control, having to pay a cost for power, etc.
The vague "primal" or "divine earth-mother" motif on the druid puts it closer to this free-range all-power, no-responsibility mold. If it is just a magical bunch of rock without the divine earth-mother personification, well, the planet is so ambivalent to cruelty, destruction, and selfishness it's basically a blank check. A personified planet-deity has an agenda and might (*gasp*) place some sort of restriction on how destructive and self-centered the PC can be.
- Marty Lund