There are a lot of things that gets me to subscribe; cleverness, good art, interesting story, funny or dark humor, characters, etc.
There really isn't much that can get me to unsubscribe, but the biggest one is the lack of updates for no apparent reason (I'm looking at you Fisheye Plaecebo!). I understand if the creator is busy or something and just can't get to it, but please inform your subscribers that updates will be infrequent or that there would be a hiatus, or else I'll be feeling like the series is kaput or just taking up precious space on my reading list. Even just an announcement of "indefinite hiatus" is better than just disappearing off the face of the earth.
Similarly, I dislike if a comic literally just up and leaves. Some series like Awaken, Berds and Nerds, and Heroic Villain were once a thing on Tapastic, going places, but for some reason they decide to leave. Some series might have legit reasons behind them for doing so, but I feel that if you have followers on a certain medium, don't be disloyal to them by leaving them behind to other places.
Finally, I really dislike a consistent lack of decent updates even though the creator has a buffer. Some series where I really like the art, Death Sin and Acolyte Hunters, are terribly slow in their pacing of story or the story isn't going anywhere. For Death Sin I liked the story at first, but now it's been giving me some very confusing updates, each being one page at a time and really nothing has been going on. Same with Acolyte Hunters. It's been twenty or so pages in but has still yet to do something interesting with the story. They're both in filler mode, building up tension for the conflict, however that's all its been. I can only wait for conflict or decent story telling for so long.