The first comment is pretty interesting.
'My suggestion is to drop the "Throw away your USB drive" tag line and use something else... it will just muddy your vision.'
He's more or less correct. 'Like a USB' is a bad analogy. Dropbox only replaces some of a USB's use cases and does lot of things that a USB doesn't. OTOH, he's wrong because there is no other 3 word sentence that could have done a better job. 'Like a USB' is probably the best starting point even if it only gets across 25% of the message because 25% is better than nothing. 25% (assuming it's the right 25%) might get the user to install it. Then they might get to know the backup, file sharing/sending, versioning, or whatever subset of functions they use.