Vote up is a tool used by members to make comments more noticable. If you go under a game and you see a comment you think deserves a merit (or a comment that goes into great detail of game improvement) then when you vote it up, the font of the comment will get bigger. You will not see a change but all the mods will see the font size change.
I am not sure if voting up is any different now but that is the way I was explained it worked a while ago.
