Subscribing
1) Usually longer ongoing series are easier to catch my attention, as I absolutely hated episodic or short joke comics/TV series when I was a kid (everything happens and resolves in 1 comic or episode where nothing is ever connected and pretty much if you watched 1 episode you watched them all) or short little comic strips that end with a joke. This has continued with me (though not as bad as when I was a kid) I'll still read a few joke or gag comics here an there, but they overall are not very appealing to me.
2) Characters and story that I find appealing. If the stories is boring (or illegible... because I'm pretty sure some people use google translate for the English versions of their comics and call it a day) or the characters I just don't give a shit about, then a subscribe is not going to happen.
3) Art I'm actually not too picky with because I actually like seeing artists improve overtime, but I at the least want to be able to tell who is who and what is going on. Story is more important to me than art.
Unsubscribing
I don't often unsubscribe, but there are a few instances when I do.
1) I mentioned before that I'm not too picky about art, HOWEVER there is an exception to this, if artists show no improvement in their art or the drawings progressively get worse then yeah... it's kind of a turn off and you better have a ****ing good story at that point or I've lost interest.
2) It's a gag a day that's gotten stale and just no longer humors me, I'm already hard to hook onto those things, they actually do not have to do much for me to lose my interest.
3) They haven't updated for over a year, the only exceptions being if I know the artist personally (or is Hunter x Hunter.)
4) Graphic sex or fight scene that just makes absolutely no god damn sense and literally has no point in being there and/or came out of nowhere. Like, the story and character would be/act exactly the same without it. So why...? WHY?
5) The artists does a 'leaving' or 'I have lost interest in this comic and have stopped working on it indefinitely' 'cause there's just no point in still keeping subscribed at that point.