The rarity and power of magic will indeed be key. One reason I picked D&D magic (besides the familiarity) is it's both common (anyone with an above-average Int can apparently learn it) and extremely powerful for magic systems. Something like Gloranthan magic would have a very widespread, but much less drastic effect, and Ars MAgica style magic would probably disappear. ;')

And I do want to read your Balanced Sward trilogy one day. It sounds interesting.