Dazed condition would work. Stunned perhaps for high level, stunned is a very powerful conditions though. charm might be duration... charmed for 1 hour or if they make it charmed for 1 round. Binary on/off spells are pretty boring...
Hopefully they don't add "stunned for 1 round" as a secondary effect on a save. That's effectively a no-save stun, which is incredibly powerful.
The one good thing I like about HP thresholds over HD thresholds is that you can whittle down the HP over time, which means you can work toward making your spells effective. It enables tactical play and leads to some fun combat, although there is still the problem of beating on the baker until you can charm him.
I can imagine some mechanics that might be more satisfying from a gameplay perspective, but everything I've come up with so far adds complexity and book-keeping. For example, imagine if characters had another stat, Mental Resistance Points (MRP). Then some spells might have a MRP threshold beyond which they are ineffective. But in this case, if the subject is over the MRP threshold, the spell could do MRP damage. That way you could whittle down a subject's resistance to mental magic, until eventually you can toy with him at your leisure. In a similar fashion, physically-debilitating spells might still have the HP threshold, but could do a small amount of HP damage if the subject is over the HP threshold. In this context, beating up the baker so that he is more susceptible to, say, Contagion, actually makes sense.
So far, the compromise I like the most has been the 1E saving throw progression. Keep in mind that some of the reason that spells were still effective is that, even at very high levels, lower-powered creatures could still be dangerous. So even though your Confusion spell might not have any effect on the fire giant High Priest, who only needed a 2 to save, it would still probably catch a number of the fire giant grunts that could otherwise make a serious impression on your 20th level party. In 3E, those fire giant grunts would be superfluous fluff, so nobody cares about confusing them. In 1E, confusing the grunts would be a big deal.