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In both the new playtest packet and the previous one, alot of spells use what I call the HP threshold mechanic. Example spells include Bane, Charm Person, Command, Ghoul Touch, Hold Person, Ray of Enfeeblement, and so on. These spells either won't work at all on targets with more than a certain maximum hit point total, don't allow a saving throw to targets with fewer hit points than that threshold, or have reduced effects against creatures with hp maximums above the threshold. This is a terrible game mechanic and I strongly urge the designers to stop using it. Here's just a few of the reasons why this mechanic is bad for the game:
  
* It forces players to guess at a monster's maximum hit point totals (or worse, resort to using metagaming or peeking in the monster manual). Guessing wrong and attempting to use the spell on a creature with too many hit points usually means you waste the spell with little or no effect. That's frustrating and not fun at all, and not for any good reason. At least when a creature makes its save, you knew there was a chance that you might have accomplished something, so you don't feel like a complete idiot for wasting your turn and prepared spell for nothing.

* Many of these effects don't even allow a saving throw, even though the conditions or penalties they inflict can be quite severe. A good example of this is Charm Person, which offers no saving throw if your hit point maximum is less than 25. Another example is Command, which offers no saving throw if the target's hp max is less than 30. This isn't even save-or-suck. It's worse! It's "have enough maximum hit points or suck," without a saving throw or any other way to resist at all.

* As characters increase in level, these spells eventually become worthless, since the threats the PCs will be typically fighting will have maximum hp totals above the limits of these spells. Why should some 1st level spells, like Grease, remain useful forever, while spells like Charm Person eventually become useless? This isn't the first edition of the game to have this problem. We saw the same problem with spells like Sleep in 3.x, which was overpowered at levels 1-5 but then became useless later because it couldn't affect creatures with more than 5 Hit Dice. The one extreme does NOT balance the other. It is NOT okay for a spell to be overpowered at low levels or any level. Making it useless at higher levels does NOT balance it out, any more than wizards being frail weaklings at low levels was balanced by them becoming godlike at high levels in past editions. Bounded accuracy helps somewhat to alleviate this issue, but it only delays the inevitable. Monsters may stay releveant longer, but they don't remain relevant forever.

* It punishes some classes more than others. Since your hit points are determined by your class, the result is that wizards end up being more susceptible to these spells than fighters are, and so on. That's ridiculous. At least Hit Die limits treat all classes fairly (not that I'm a big fan of that mechanic either).

*[EDIT] This mechanic is contrary to bounded accuracy. One of the design goals of this edition is that players and monsters don't get large bonuses to stats, attack rolls, saves, AC, etc. just because they're higher level. This gives lower level characters a chance against higher level creatures, but also allows lower level monsters to remain somewhat relevant threats against higher level characters. But there is one stat that does still greatly inflate with level - hit points. And by tying that to these spells, they alone are stuck with the old level paradigm, even though it clashes with the rest of the system.

This mechanic is awful and needs to go. Any spell, ANY spell, that debilitates a character in any way needs to have either an attack roll or saving throw, as appropriate. It's simply not okay to have such spells automatically succeed or fail. If a spell like Sleep is just too powerful as a 1st level spell without a hp limit, well, then make it a higher level spell! Sleep in particular has always been problematic, being effectively a low level save-or-die. Maybe it's just not appropriate for a 1st level spell. Maybe it never has been.

There are also many other, far better ways of balancing these types of spells. Players could be given multiple ways to break out of their effects, not the least of which should be a saving throw every turn in combat to shrug off a nasty effect. Spells like Charm and Suggestion can have reasonable limits on what they can make people do (like not being able to make them commit suicide or attack their allies) so that they aren't effectively save-or-lose. There's even the possibility of giving higher level beings a bonus on their saving throws rather than granting them outright immunity. There are plenty of other options. Almost anything is better than the hp threshold mechanic they're using now to "balance" these spells.

Though I'm usually one to play devil's advocate, in this case I agree with you 100%. I hated the HP threashold mechanic even back when it was called Power Word (^{_{insert effect here}}).

Unless I'm mistaken, saves based on spell level was the approach done with 13th Age (except given how it uses the 4E unified attack system, it's static bonus to hit unless you use a higher spell slot level).  In short, the idea's pretty solid