I'm not that picky. I read practically anything. Though often it's the genres that'll get me to turn away from the manga sooner rather than later, if anything. Like, I can probably count the number of actual tragedies I've read on one hand (I dislike that genre). Not particularly a huge fan of ecchi, though there are some stories that I enjoy that include it. Generally slice of life stories don't catch my interest, but I don't really have anything against them (the plot summary isn't enough to draw me in). And I don't usually read much horror. Oh, and I think I've read a grand total of three GL stories (girls' love aka shoujo-ai), all of which were one shots (found out that one of them got serialized and has more chapters, but haven't been interested enough to go back and read it). But really, anything else is pretty much fair game. Even hentai.
The next thing that turns me away from reading a manga is if I recognize the mangaka and I have them flagged in my head as someone who completely lacks creativity. I mean, when you read two or three or more of their stories and you realize that all the main characters look the same, and all the main love interests also look the same, and both the main characters and the main love interests have the exact same personalities in all the stories, and even if the plot starts out completely differently and with a very interesting idea, by the end of it it has the exact same outcome as all the other stories, and in general if you were to start another one of these stories done by this mangaka you'd be able to predict the entire story a couple of pages into it, then really, is there any wonder as to why I want to avoid boring myself to death and wasting my valuable time reading the recycled unoriginal story over and over? (For the record, stories that fall into this category are done by: Hwang Mi Ri, Han Yu-Rang, and, although I haven't personally experienced it for myself but I trust my sister's word, Mayo Shinju is also on this list)
As for demographics, I technically fall into the "josei" category, however it's the demographic from which I've read the least amount of stories. There are some stories in this demographic that I absolutely love (+C: Sword and Cornett, Are You Alice?, etc), but for the most part, they generally don't appeal to me. As for shoujo, shonen, and seinen, I have no preference really - just depends on my mood, I guess. I've probably read more shoujo than any other genre, but that's 'cause it's usually pretty easy to read.
Of course, good art enhances the experience, but I don't think it's a complete be-all-and-end-all for me if it's not as good (though, there have been times where I've had difficulty deciphering exactly what is being portrayed on the page, and in those instances I wondered whether it was worth continuing to read... some people are good at drawing action and still making it legible, while others might think they're good and just... aren't).
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