But after playing two sessions and more careful reading, I fail to see the MM's advantage over Ray of Frost save range. The slowed ailment more than makes up for that in my eyes.
Slowed is a pretty marginal condition. If there's an enemy 5 or more squares from any ally, and no ally moves up to it after it's slowed, you've saved the party an attack. It's handy for slowing down a fleeing enemy at the end of a fight. It /should/ be good against skirmishers, but too many of them have movement powers that aren't based on their speed...
Against that, Magic Missle has:
double the range
1.5 more average damage (5 vs 3.5) (and more consistent damage since it rolls two dice rather than 1).
Force damage instead of Cold (both Force and Cold have some feat support that's pretty good - but Cold is resisted more than Force)
Target REF rather than FORT - this is a slight advantage, but FORT tends to be a good non-AC defense more often than REF, and, when it is, as in Brutes, particularly, it can be quite high.
Useable as a Ranged Basic Attack (not as important as an MBA-equivalent, but there are times when you may be restricted by a condition to making a basic attack, or when you're granted a bonus basic attack, or gain a bonus /just/ to basic attacks).
Specific feat/item/power support that expands it's usefulness (some very specificallly /just/ from MM).