Hipchat just feels really enterprisey. Like those CRMs when you sign up for a trial then suddenly some guy just schedules a meeting - would you be free to Skype Monday or Tuesday? While I don't know whether Hipchat actually does this, part of the reason I don't want to sign up is because there's some worry that some guy in a suit would be sending me lots of emails about when a good time to meet is.

Slack just feels casual, like Trello or Google Docs.