Others have already mentioned most of what I like, but I'm also a fan of the muting. If an alert has already fired, additional instances of the same problem don't result in a new alert. No more opening your mail client to find 5000 emails with the same stupid "An error happened" subject line.

Integration was dead simple as well. I couldn't use the official client (because reasons) and it was only a few lines of Python to fire alerts from TIBCO Hawk.