I've been mulling this one over, and I think I agree with Fan most of the way. I don't think reducing HP as you're describing is going to break the game. However, I'm not quite certain we're going far enough to "buff up" the humanoid monster in compensation.
More damage is good. What about "do an extra 1[W] die of damage when bloodied."? That might help address "grind" by making it feel like the stakes go up when the bad guys' chips are all on the table.
More numbers gets dangerous: an unlucky party can get overwhelmed quickly with bad dice luck, too much Combat Advantage against them, etc .. but maybe humanoids are more likely to capture than TPK so that's not an issue.
It places more emphasis on powers which hit a number of guys for relatively weaker damage than hitting one guy for strong damage, but I think that's what you want. It does encourage use of Controllers and area-Strikers more than Defenders and single-target Strikers; again, that may be what you want.
But I'm wondering if what you really want, Rachel, are two-hit minions, minions with resistance, etc ... reducing the number of non-leader humanoid bad guys entirely, and leaving you with a typical fight including one or two Brute (easy to hit, high HP) bodyguard/lieutenant types, one difficult boss, and a number of low-HP-but-not-quite-one-shot-wonder bad guys.
Having faced that as a player, I have to say I really like the way it "feels": somebody has to square off against the boss to keep him from running rampant on the squishies, and whoever squares off against the boss feels like they got an epic battle with a legendary villain .. while the rest of the group gets a lot more satisfaction from killing two-hit minions than one-hit minions. I'm not really sure why that is, but it is.
Not saying the "reduce HP is the wrong way to go"; as we've seen, there are plenty of versions out there (1/2 HP, 2/3 XP; 1/2 HP, x2 dam; etc) where people are running that, and there's no reason you can't just slap one of those templates onto humanoids and leave it at that ... I'm just wondering if your goal is "making it feel like a sword blow that 'hits' really connects hard," rather than explicitly "hey, humanoids have too many HP", and if that's the case, an "encounter design" solves the same issue as well as or better than "mod all humanoids".
In fact, maybe its an "apply as desired" attribute, e.g., treat each humanoid race differently. Goblin leaders, Kobold chieftans, etc? Fewer hit points, fine. 7' tall gnolls? Leave 'em as is. Orcs, maybe some tribes are smaller than others.
Just chewing on it .. I'm sure this is going to be a problem in my campaign when we hit paragon + levels, so I'm still toying with "how do I want to solve it" myself.