For Centralia, people often say they like:
Backgrounds-- A lot of people have told me they really like the background/environment art of the comic, which is nice, because I spend a buttload of time on them.
Midori-- Which is great, considering she's the main character! It would be unfortunate if people couldn't stand her. A lot of readers have indicated that they can empathize with her, which I'm happy about.
Mystery-- A lot of readers seem to be enjoying the mystery of the plot, and get excited when new hints and information are dropped. On the flip side they get upset if I tease them with information but don't reveal big secrets |D Haha...
My favourite feedback is when people comment on the story or writing. I appreciate feedback on the art too, but I'd rather focus on having a compelling narrative than just a bunch of nice artwork. So when readers tell me they're enjoying stuff about the writing, it really makes my day. :>