By providing actual input.
It doesn't even need to be in the traditional format as long as the input's useful. Ex. "Your map is boring to me. This is because you use the same patterns over and over and don't pay as much attention to the music as you could, which probably would make the map funner. Also, 4 and a half minutes of the same song seems overkill to me, I would consider cutting it to a more reasonable length. And no, no one cares about your desire for purism."
As long as it's not like, "I LOVE IT" or "THIS MAP SUCKS" without anything even vaguely constructive in the post, then it can often be useful and kudosu-worthy.