I find the design and polish of Slack to be significantly superior to that of HipChat. Feature-wise though, I'm with you and couldn't pinpoint the differences.
I too generally find Slack to be a more appealing user experience, though along with others I find the advantages over, say, Skype plus email to be fairly nugatory.
Because I work on a bunch of different teams for different organisations, I currently have open Slack, HipChat, Skype, and Apple's Messages, and I have to use Google Hangouts (which periodically completely kills Chrome for me) for a daily standup. I sort of miss those happy days when you could just use Adium for everything.