People who have significant gripes or kudos are more motivated to find out how to give feedback, and make the effort to do it, but I think Arisia benefits from getting more "mundane" feedback about the less exceptional stuff. Which panels did you enjoy and want to see happen again? Were moderators effective or ineffective (sometimes a good panelist turns out to be a less than great moderator, and it'd probably help Arisia to know - the staff can't personally attend every panel) ? What did you think of the changed layout of the dealers and art show? Or the mix of dealers? Did you see a performance you found very entertaining? Is there somewhere in the event space where an additional sign might have been helpful or saved you time? Stuff like that.
See Arisia's contact page
for how to submit feedback by email or web form.