I'm currently making the switch from Squarespace to Posterous, which is reminding me: in the future, when every service of any importance is a backend (or 'API,' if you'd like to call it that) for other front-facing client platforms (or 'applications,' if you'd like to call them that), data portability will become of critical value.
That is to say, people will want to be able to move their information seamlessly from platform to platform, as they decide which platform best fits their needs. Think of that short time ago when people started becoming fatigued by all the profiles they'd have to build every time they joined a new site. Facebook Connect/Twitter/Google/Myspace/Yahoo/HipsterRunoff Connect have all served to solve the 'profile building' problem, but there's also the problem of transporting all the data we've collected into sites like single-serving tumblrs, personal analytics on Daytum and the like, and the massive numbers of tracks played/collected/liked/shared on sites like Hype Machine and Blip.fm.
Posterous has made this exceptionally easy for blog data, which is dead simple these days because of web CMS standards. Applications and platforms that follow the same principles, applied to non-CMS data, will have that Apple-esque ephemeral touch of 'useability' that makes people switch platforms.
