Art: A style that fits the story and mood of the comic. I like art that looks competent, and does more than just illustrate the story. That is, I like it when the art is used to enhance the mood, explore the setting through background detail, etc. It's amazing how many artists neglect to use their art to do more than just show the characters saying/doing things.
Plot: Solid characters (but I think that's a given) and a sense of mystery in the plot. I don't care if it's a comedy or an adventure of a detective story, if I feel like I know everything and can predict everything, I just get bored and stop reading. I like it when stories keep me guessing. And I mean for the long-term; I don't particularly care for "oh noes, a shadow appeared! Who is it?!" cliffhangers. They're not bad, but they're not enough. Of course, at the same time the plot shouldn't just be piling questions on, it should also be answering them.
Consistent, living worlds. I don't care if it's Planet Zed, Squireshire or 1930s New York City, I want the world to have unwritten, unbroken rules and I want it to exist independently of the characters (unless that's a plot point). The same goes for the minor characters that populate these worlds - I want them to have their own goals and at least a semblance of a personality. In short, I want settings that are worlds populated by characters, not just backgrounds.
I like it when a plot is more or less compact. That is, everything given focus is important, there are no "fillers", every piece of information has relevance. As much as I love world-building, for example, I don't really want to hear about why the sky is red if it has no bearing on the plot at all. Save that for the extras. At the same time, a story should not be austere, where the only things said are relevant to the plot and nothing else. That is, a story should be written in such a way that the setting, character histories, etc are integrated into it and are not interchangeable.
Genre, twists, romance are irrelevant to whether I think a comic is good or not. They are relevant to how much I enjoy it, though. For example, I enjoy a few comics I don't consider good, and there are lots of comics out there that I think are good, but I don't read them because they're just not my thing.
Now, my preferences? These are in addition to the above, so I won't repeat myself.
I like serious stories over straight-up comedies, as I like my humour in small doses (unless it's Mel Brooks-calibre humour). I love it when characters have interesting, developed relationships, and I don't mind if those relationships are romantic, but I don't care for stories with romance as the plot. I tend to view relationships as an interesting side effect of other things happening, they are not in themselves enough to keep me interested.
Darker stories grab my interest better than more optimistic ones, not sure why.
I read a lot of fantasy and horror stories, but I'm not sure if that's because I like the genre or because most dark-ish non-romance stories with interesting worlds tend to be fantasy/horror xP
I have a few guilty pleasures in comics too, but I think I'll make a separate thread for that.