XenForo's alert system kicks other forum engines' butts Muay Thai style which means HARD. XenForo is in a way reinventing forum interaction. For example, take quotes. If I wanna make sure the user gets my message I just quote him or her. Voilà! If I don't really care about his or her response, I just reply without quoting.
All of a sudden, quotes became an invaluable tool for me.
Let's see how long it's gonna be before vB and IPB adopt XenForo's alert system.
I find quote alerts very useful from a moderator stand-point. When a question is asked, members can view their quote alerts to see if a staff member replied (if they choose to receive alerts for quoting). I can view mine as well to see if anyone quoted a post I made so I can follow up if need be. An unexpected, and very valuable feedback tool in my opinion.