For me it is the Social Networking features. That would be the alerts, likes, and the profile follows system like Twitter. So what are some features of Xenforo that you like?
Alerts, alerts, alerts. Probably my favorite XF feature by far due to its ability to engage and encourage your members to keep posting.
Alerts are my favorite feature. They keep you up to date on the threads you post in and people's response. I've noticed it helps promote activity as well.
I like how when you're logging in you can login by Email or Username. Its like a back up if you can't remember what username you used.