I've decided that the single most important issue in writing is making the reader care. It's not something I've put a lot of thought into, to be honest. It came about through working in my critique group.
I've never worked in a crit group before...and boy, do I wish I'd started sooner. I don't know why I didn't before. Guess I couldn't really think of how to find one, and didn't bother looking to hard for one. As it is, Wunderkin sort of fell in my lap, and nothing--not my mama saying nice things about my writing, not my college professor saying mean things about it--nothing has been as beneficial as swapping 50 pages with 5 other people of all ages and tastes and seeing what they think.
I sent them Babbletongue first. I've had issues--it's a pretty unique, creative idea, but people either love or hate the MC, Mina. My crit group was mostly on the hate side--they didn't "get" why Mina was acting the way they did. When my turn came around again a month later, I sent the first fifty of The Red Thread. They had trouble with the in media res beginning.
To be honest, though, I didn't really get what was wrong with my writing, until I started analyzing theirs. I'd read one of their works and write in a comment--"Why's this character acting this way?" or "This is too random!" or "I hate this character!" These characters were doing such random things for no apparent reason...and when things happened to them, I didn't care one bit!
...and then I realized what was wrong my characters. I had them doing random things for no apparent reason and had yet to realize that the reason was because I'd not made my reader care yet. This may be something all writers struggle with, I don't know. For me, I knew my characters, I cared about them, I knew why they acted the way they did. But I forgot about making the reader care. I forgot that just because they were on paper, they weren't alive. I had to show my reader what made my character special, or they would never care what happened to her, never understand why she acted--or didn't.
My problem was that I focused on the action of the plot (I love a fast pace), but I let the characters be left behind. It's just as easy to lose readers by focusing on the characters and ignoring the plot. This isn't really an issue of character or plot--it's an issue of creating sympathy--or at least empathy--within the reader for something in the text. Whether the reader cares about what will happen or cares about the character, the important thing is that the reader cares.