Zappable/applied effects could be a nice distinguishing feature between magic weapons and artifacts.
I'm not too keen on adding more effects to magic weapons, as they are already quite strong and useful, and adding more effects to them would make them overshadow normal weapons way too much, IMHO. Flaming swords give you light and fire damage which is not blocked by armor, thunder hammers have very strong lightning bolts that will kill everything quickly if you can survive them yourself, whips of thievery are just badass, etc. They are all quite useful without being overpowered, and even those that can be a bit tricky to use (staves of wondrous smells) are actually quite effective if you're careful.
Giving zappable effects to artifacts, though, would be a nice way to make them feel even more special. I like that.
chaostrom wrote
For that matter, how complex can a weapon effect be? So far all existing effects are singular, what about a rare, time-based weapon that can slow an enemy while hasting you at the same time? Is that possible?
That would be rather easy. Chilling axes already slow your enemies, you could just add:
Hitter->BeginTemporaryState(HASTE, 100 + RAND_N(200));