...like a hot rock: lack of openness/integratability with my other social sites. For example, requiring users to authenticate by default to \*read\* content on generally public social sites and and not opening up interfaces to integrate with other social sites is a major PITA for users and the first site that manages to effectively integrate my social network activities without limiting access to my friends social network activities, wins my sign-up.
People are getting fed up with shifting from one social site to the next and inevitably pitch camp where their friends are. Makes sense. But if sites were open and enabled \*easy\* integration (via open APIs & comprehensive UIs that enable easy use of the APIs, etc.), this wouldn't be an issue.
It'll be interesting to see what Plaxo releases on Monday