Hmm... It is okay for green to get Pacifisms from time to time, and this is a nicely in-flavour one. But MaRo is of the opinion that it's best to avoid it where possible, because green and white are already the two colours with the most similarities, so it's better to play up their differences. I would tend to agree, especially if you're in a set with only four colours rather than five.

You could flip it around: "Enchanted creature has protection from creatures with lower power"

Fills almost the same niche.

Or you could compare it to blue's lock down effects instead of white's. "Enchanted creature can't untap unless..." will generally give you the same results.

On another note, people will mis-play this (or at the very least wonder why you'd ever play it). Obviously, "you" always refers to the enchantment's controller, so you can play this on an opposing creature to lock it down if you've got bigger stuff; but a lot of players will think it checks the power of the enchanted creature's controller's creatures (as if the enchantment granted the ability).

"Enchanted creature doesn't untap [unless blah]" does still feel like Pacifism, and does still contribute to green and white feeling too similar.

I've always thought of blue and white as being more similar than green and white, but green and white are probably more similar than red and green.

Oh, and Chris... you're probably right. :-(