That's really a tough question to answer, because there is no black and white answer. Everyone views things differently. I'm a firm believer that it is nearly impossible to get your original tone and intent across in text.
I work in an industry where if something goes wrong, or if something doesn't go right - I get harshly criticized for it. I also always recieve well intended criticism - but 98% of the time the person comes off as a raving jerk.
The only 'solution' I may have to your question is when you criticize something - maybe think up a solution for the issue. I would never go to my boss with a problem I don't have an answer to and generally I take the same approach with any other issues I have.
So in reference to the Anime Milwaukee Cosplay Contest - along with your criticism you may have been able to pass along some valuable information on timing, where the contestants should move to, and how to keep the audience interested through a 'boring' part; and you could have been seen as wanting to help instead of being a 'jerk'.
Sorry that comment was so long but I hope it helps!
