There was a discussion on Harm over at the board-that-shall-not-be-named, and I'm pulling a few comments I posted over there. To give a frame fo reference, I'm of the "don't house rule it if it doesn't cause problems in your game" camp, and have never had any problems with Harm (1e, 2e, or 3e), nor have my players ever expressed a problem with the spell being used by their opponents. Your group may have a different outlook.
Someone said it was too easy to get close to opponents and deliver the touch attack. Dragons got brought up, as they often do in harm discussions (and just as often, SR is "forgotten" in such arguments) My suggestions:
Attacks of Opportunity, Reach Weapons, Spell Resistance, Readied Actions, non-Armor AC bonuses (there are quite a lot of them out there)....
If a Dragon in your campaign lets the Cleric walk up and touch it, it deserves to die, and the DM deserves a smack for handing over the dragon's XP and treasure to the PCs. (was in reference to Dragons fighting intelligently - i.e. not letting Fighters get full attack actions against them, not letting Rogues flank them, using their mobility, breath, and spells strategically)
Someone else commented that there are no spells of 7th level or below that can screw over a PC without a save. A partial list:
Antimagic Field
Blasphemy
Creeping Doom
Dictum
Enervation
Force Cage
Geas / Quest
Holy Word
Mark of Justice
Power Word, Stun
Word of Chaos
Granted, most of these aren't as nasty as Harm, but there are many ways of putting a hurting on an opponent that provide for no saves.
At 12th+ level, the game gets downright deadly - anyone who's taken a good look at the type of challenges a higher-level party faces knows this to be true. It might be LESS deadly now, HP-wise, since you're still getting Hit Dice at that point (and CON bonuses are better) but, as they have been in previous editions, the spells are still something to worry about. Some might not kill you outright, but at that level, even being unable to act for one round can easily be fatal.
And if you're relying on armor alone to protect you (re: touch attacks), rather than good strategy and defensive magic, you WILL get yourself offed. Past 8th-12th level, you're (typically) one of the biggest bad-asses in the land, and the challenges you face start ramping up quickly.
Play with Harm as-is for a while and see how it works. If it doesn't work out in your game, I'd suggest allowing a successful Will save result in the target's Hit points being halved.