Reminding people of the dreaded windows "blue screen" and the amount of malware and viruses that tend to be going around is a sensible way to encourage new users to use Mint I think. By virtue of a dual boot if nothing else.
Given that they could retrieve certain files if they ran into problems on the windows side, this is a genuine "unique" benefit rather than a "give it a try you might like it" gamble.
After all, many are happy with Win 7 and won't want to try it anyway, "if it aint broke" etc.
But it will break.
And that's where mint steps in for them. Sure they don't know what they're missing. But a few well placed tutorials of "how to recover your dead windows machine without the disk/licence number" might help? Or even a "why you need to dual boot" set of youtube virals?
I notice that Radio advertising has also been mentioned on here too but it's hard to think of a way to phrase an ad without sounding either paranoid or a local nut hung up about windows. Or both.
A backup system is a good "foot in the door" for new users perhaps.
Anyway, all the best.
PS The "online test drive" tour that Ubuntu offer is also a great tool I thought.