For me personally, it has to be a good story for me to keep reading, and if it has not grabbed my attention and interest by about the 10th page, I have to stop. I am the same way with novels as well, and I know that if you are patient, the story may all of a sudden become fantastic. But that is not the way I am. However, on the art side, if the art is not good, or even okay I will not give it a chance, as I look at the art before reading.
But what makes the art good from bad? What is the line that marks readable from unreadable? For myself I like smoother lines compared to rough, and surprisingly, I prefer black and white to coloured. Don’t get me wrong as I like colour every now and then, but when its from older comics, like those that we have read (X-men for example), I find the color very distracting, to the point of causing headaches, and it was very hard to read. The art also needs to have detail, to some degree, and too much becomes a distraction from the story as well.
The balance in the middle is what art needs. There is also the font choice, which even though it sounds small, its also important for me personally. If I have to squint or re-read a sentence, because of the chance that I read it wrong because of the font, then there is a problem. The illustrations themselves don’t need to be bad, to make it bad art, as it’s the whole package of the layout that make the art a make or break for me. If the layout is nice, words are legible, and you can easily make out the illustrations, then it has hit the ‘okay’ line (for myself, as others my have a different standard). If the art goes beyond this, then it hits good, or great artwork.