You said the spells deal NONLETHAL damage to the caster.....? Right, so Mr. 1st-level Wizard fresh out of the academy can then proceed to, if stricken by a sudden fancy of madness, obliterate the academy and everyone in it by uttering a *power word - fire* to deal 1 million damage, and heck while he's at it, why not blow the planet to smithereens? Because, y'know, there's no limit. And then Mr. 1st-level Wizard will wake up when all that nonlethal damage has healed, because y'know, being nonlethal, it couldn't possibly kill him, no matter how much real damage he causes with the power word.
And, of course, any amount of normal damage healed in D&D also heals an equal amount of nonlethal damage for free. So, Mr. 5th-level Wizard uses a *power word - health* to heal not only 10 damage to himself, but also 10 nonlethal damage, allowing him to cast many more spells that day. Since there's no limit to how many Power Words they can use each day, they can use a Power Word - Fire to deal enough damage to enemies such that the Wizard is left at 1 point of nonlethal damage away from unconsciousness, then use a Power Word - Health to restore himself fully, and continue the carnage next round.
My suggestions to ensure such craziness won't happen, and to better ensure balance:
*Up the hit dice of wizards and sorcerers to d6s, they'll need it, and besides, their niche is no longer so special since even mundane warriors and thieves can learn magic, even if only at a slower pace, since those warriors and thieves will have more hit points to throw away on spellcasting, getting bigger/broader/more frequent effects.
*Set up rough paremeters for how much of an area, mass, weight, or value (in CP/SP/GP) that a Power Word can affect based on how much feedback damage it deals the invoker; ya don't want someone with Power Word - Gold to go turning every pebble into a gold coin just because there's no limits in the game.
*Say that the feedback damage suffered from a spell is only nonlethal if it does not exceed their caster level, perhaps, and put a solid limit on their Power Words saying that they cannot invoke a Power Word if it would deal them more than twice their caster level in damage; thus, for instance, a 1st-level wizard could deal 8 damage with a Power Word, and so could a 4th-level fighter. You may want to double this limit (to x2 for nonlethal and x4 for lethal, respectively) depending on the scope of your campaign.
*Say that classes which normally lack spells, such as fighters, have an effective caster level equal to 1/4 their total level in such non-spellcasting classes, for purposes of using Power Words.
*and, well, I'm out of time, maybe I'll add something later if it comes to me.