On Grassblades, it was something that pretty much amounted to
"This is a nice comic, but your updates are too short. Make them longer. Also, I really don't like the way you draw faces, so you have to fix that. If you don't, I have to stop reading."
... and as rude comments go, it's not that bad. It's just the sense of entitlement that makes me squint. I'm doing this comic for free, buddy. You don't get to dictate how I do it, unless you're paying me for the privilege.
On my holiday oneshot, I kept getting a bunch of fairly graphic sex-jokes in the comments, which I actually deleted as they made me uncomfortable. They weren't rude or insulting, so much as they were very out of place.