First, get rid of anonymous opinions. Anonymous allows DDO members to sign off and vote again. Other sites, like YouTube comments and Twitter, require an account, so why not DDO?
Second, let us change our opinions. We are human beings, we change our minds and opinions all the time.
Third, reduce the "word requirement" from 50 words down to 25. Often users write stuff like "hippity hop", "I will now waste 25 words, I will now waste 25 words", and even "aaa aaa aaa aaa" just to get their opinions posted. The person who starts the opinion could possibly set their own rules.
Cultivating an atmosphere appropriate for a debate is a time consuming process. The rules developed are once that have been there for some time and for now work well as guidelines that contribute towards the coherency of the debate going on on Debategraph.org. Rules can be changed once it feels like conditions are right for doing so.