There's also a hint in his post about trade-offs within each spell. For example, Sleep is effective against low HD creatures, but not against those with higher HD or against creatures that just don't sleep, such as the undead. But the "newfangled" Greyhawk spells often don't have that feel; there's no downside to Magic Missile, it's just a way to add distance damage to 1st level M-Us.
So how could we fix it?
First, let's look at the spell as written:
This is a conjured missile equivalent to a magic arrow, and it does full damage (2-7 points) to any creature it strikes.Most of us focus on Magic Missile as just another attack spell, but what if we interpret that first phrase literally: the purpose of the spell is to create a +1 magic missile out of thin air, in case you have no way to damage a creature hit only by magic weapons. There's our trump that we're looking for...
Later versions of the spell emphasize that the missile strikes unerringly, perhaps even with no saving throw, but we don't have to follow that rule, now that we have our trump feature. The new version of the spell creates one or more small, throwable magical projectiles, but doesn't aim or fire them for you; use the standard attack roll (at +1, because it's a +1 magic projectile.) You can throw one missile per round, and they will last up to 10 minutes.
A different option would be to drop the attack roll or saving throw, but require an actual physical missile. In other words, the spell turns one to three rocks, arrows, or other mundane missiles into one-shot +1 missiles, then causes them to fly out of the caster's hand towards a target.
Either way, now that we are thinking of this as a physical attack improved by magic, we can see that there is a trade-off: Magic Missile won't work against creatures that are immune to all physical attacks, even those from magical weapons. So, ghosts can't be hit by Magic Missile, unless the caster is ethereal.