Typically I'm not really one for vocals, though I can tolerate them. Having an impactful/catchy melody is of course necessary, but if that were the extent of my criteria I'd like half the songs that have ever been made. I like songs that can reciprocate a certain mood that I'm looking for, which is made more difficult without vocals. More specifically, songs that accompany something else, like a scene or a story. A song on its own simply doesn't have the means to be great, in my opinion,