Just to grab me, you have to:
Make it look and feel like magic.
Work flawlessly in the first 10 minutes. If you can’t survive 10 minutes of critical analysis, I’m gone.
Provide additional, unexpected awesomeness.
A nicely argumented writeup of Dropbox, the excellent (and excellently free) file sync / web access solution that will very unfortunately and undeservedly get slaughtered by Microsoft Live Mesh and whatever half-baked solution Google will launch at some point. (Notice how I didn’t include Mobile Me in the list?)
In the mean time, do install it, and welcome to the future. That way at least you’ll know what you’re losing in a year or two, when you’re dragged to another, more famous service by your clueless social circle. (Or when Google buys out Dropbox and proceeds to sink it.)