Strong characters = good book--it's that simple. Too many beginning writers stress out trying to come up with a great plot, while the characters in the book end up being ignored by the writer...and that's a recipe for disaster. Interesting characters will hold the interest of a reader better than an interesting plot. There are writers (Michael Crichton comes to mind) who can create thin, barely fleshed out characters and get away with it, but even those characters have to have enough personality to be interesting.
Good characters are flawed. Good characters have issues, and they attract other characters with issues. Nobody wants to read about a character who lives a perfect, boring life--unless that perfectly boring life is symptomatic of a deeper problem. Good characters can be good or evil, (ideally they should be some of each) but they all have to be put into a situation that throws them out of their comfort zone--and that's the key to writing interesting stories; plot comes from the problems a character must deal with, whether it involves saving the world, destroying the world, changing the world, or simply learning how to put up with an intrusive mother-in-law.
Plot is the end result of an interesting character dealing with a challenge. An interesting character is going to tell the writer how to write--and the plot will take care of itself. Too often writers forget that, and it is a sure way to turn off a reader.