The biggest thing that comes to mind is native clients for every computer and device I own (PC with both Windows & Linux on dual-boot, Mac desktop & laptop, iPo

The biggest thing that comes to mind is native clients for every computer and device I own (PC with both Windows & Linux on dual-boot, Mac desktop & laptop, iPod Touch, iPad), complete with nice file browsers, integrated media playing, etc. I was going to put that in my original comment, but thought it was somewhat obvious: at first glance, it seems to me that this would require the most work if you wanted to actually clone Dropbox. That, and the nicely designed website, marketing copy, clever social/viral features, etc.