Being a good forum member is the requirement. I do not see what you have a problem with.
"Good" is very subjective, Someone may be a "Good" forum member to some people, And a "Bad" forum member to some others. They may be "Good" in certain areas, And "Bad" in certain others.
Humans are generally biased, They will see people as "Good" Or "Bad" based off their own beliefs.
If there is to be voting, I personally believe only a certain amount of people should be allowed to vote.
If it were up to me, I would make a list of people with the least amount of bias, And the most logical thinking, Then for each vote, Use a random number generator to generate anywhere between 5-11 people to vote for who gets the star award.
This method at least trys to eliminate bias as much as possible, The only bias is in the opinion of who are the people to put on initial the voting list, And the rest is as fair as i think it can be made.
If anyone sees problems with it, Be my guest and improve it.