In science, one of the prerequisites for proper research and reliable results is that the act of measuring does not influence the result. The same is true when you use a voting system to do the measuring: asking the right question and asking it the right way is not always easy. The way you ask a question can influence the answer you get.
So, if you ask for trouble, you will get trouble. This is particularly obvious when you are using a voting system as part of an evaluation.
Let me explain: at the end of your event, suppose you ask your audience what they feel could be improved, what they missed during the event or what they are unhappy about. Taking this approach entails a certain risk: even if your audience had a wonderful experience, they could still go home with a negative feeling.
Why? Well, it sounds harsh, but it is because you asked for it.
In your genuine and well-intended attempt to continuously improve your events, you may have overlooked the possibility that you are already doing a very good job. Asking for the negatives then might leave an impression with the audience that is not doing your event justice.
If you ask for negatives, you will always get some. It is better to ask for positive feedback and interpret the answers, and read the negatives between the lines. For example, ask your audience what speaker they would like to see again, next time. It forces them to remember the speaker that they enjoyed most, moving them to positive thoughts and sending them home with a good feeling about your event. And indirectly, the results will also tell you what speaker not to invite next time.