Let's not forget that the damage is caster level based. A cleric gets harm at 11th level, and potentially does 110 points of damage, not 150. Said cleric would have to be 15 level to do 150 points of damage. It may not sound like much, but that 40 points of damage difference is life and death for a lot of characters. I post this so that people who read this thread don't get the idea that Harm always does 150 points of damage.
It's frustrating to try and divine the intent of authors, beyond looking at grammar. Take the benchmark 6th level attack spells, for example. Disintegrate does ~77 damage (22d6) at 11th level, and 5d6 on a passed save. While it has the virtue of increased range, it does less damage on both passed and failed saves. Flesh to Stone turns the target into a statue, and takes Break Enchantment or Stone to Flesh to restore the target. Since Break Enchantment has a 1 minute cast time, it is not a combat spell, but the spell does not kill you. These two examples are fine and good, but what about Slay Living? Ahh, there's the rub. A Cleric 5 spell that kills you outright. Its Sor/Wiz 5 counterpart? Baleful Polymorph, which doesn't kill you, but renders you mostly harmless. Hmmmm. This doesn't make much sense.
Since wizards are supposed to have more destructive firepower than clerics (a claim I hope none will dispute) and one of the goals of this revision is to reduce the lethality of the game, I believe that Harm should not kill you on a successful save. I do wonder why they left Slay Living at 5th level and Save or True death when all the other good similiar attack spells don't kill you, but render you helpless/nearly helpless, but that musing is for another thread.
If Harm does kill, then man do clerics have it good in 3.5. Slay Living and Harm will kill similiar level creatures. Ah, my last point that just came to mind is that you can protect against BOTH Slay Living and Harm with Death Ward. The 3.5 Death ward will stop all negative energy (Harm) and death effects (Slay Living, which has the death descriptor). Sometimes it is best to think out your arguments before you post them...