Sleep would be an enchantment/charm/control like spell that forces someone to do something they would otherwise not do. This is going to have to require a spell DC versus ? wisdom save?
If sleep's real weakness is its hp limit- and I think we can all agree that it is- then I don't think you need a save. I like the idea of different spells being balanced in different ways.
That said, I think the hp threshold mechanic still needs some work.
I want to take this time to once again promote the idea of enchantment spells hierarchy.
sleep is the least powerful, the worst that can happen is that an enemy can take damage once
Uhh, I think you're forgetting coup de grace: you spend an action and reduce an unconscious creature to 0 hp. If it's already at 0, it dies. Sleep is FAR from the least powerful effect enchantments can produce, IMHO.
I like the idea of a wide range of enchantment effects- charmed, asleep, fumbling (disadvantage on attacks and Str & Dex checks), enraged, unobservant, etc. I hope we see a lot of this kind of thing.
hold person could be more powerful because if the enemy doesn't save, the enemy is subject to being auto hit again.
charm person is even more useful because the enemy could be made to attack other enemies, each attack could be subject to a save roll or the spell is broken
Charm person doesn't do that as is, and I love having a minor "Be my buddy" type spell available for lvl 1 casters.
Hold person traditionally does clock in at 2nd - 3rd, so yeah, I'm ok with that one being more powerful. (Note that you cannot cdg a held creature; the key word in the playtest document's description of cdg is "unconscious").
summon person is the most powerful because you can get a new henchman that way that will help you throughout the campaign.
OTOH I don't see summons as being enchantments at all.
A permanent charm, though?
Now change 'person' to 'being' and we've got a very effective set of enchantments.
It's a start, but like I said, I want
breadth, not just depth. I want
dire charm, Leomund's lamentable belabourment, geas, hope, fear, discord- all those and more.