How about having some sort of point system , that when you comment on someones riff you gain points and if your comment is the voted the best you gain more points. You can also have more levels instead of just participant. Lets say there is some sort of rewards for the highest level you can lets say perhaps get master tracks of a7x songs so that you can customise them for your guitar covers, just an idea. Or some over reward. It will give more of a incentive for people to comment and help others out and then it will help people who is commenting also. Maybe someone else can elaborate on this.