When people comment only about the story, how they think it will develop, how such character is going to react to such event and basically talk about them as if they were real people.
When there's no talk about the art or comic's mechanics, then I feel like I've won.
There's a saying in arts whenever a story is involved : "If, while watching, people can comment about how much work went behind it, it means you're doing something wrong." It means there's something missing in your story or characters if they can get out of the story that easily to thinks about the work behind it.
Then again, in Tapastic's case, this is to take with a grain of salt : as I give my readers one page/week, they're constantly pulled out of the story, unlike if they were given the full volume in one go. Also, a huge part of said readers are fellow artists that can't help but notice the work that went behind each pages.
TL;DR : I like when readers tell me they like my art but it's when they only comment about the characters, universe and story as if it were real that I feel like I've truly won.