Plot happens. Or at least, that's what they say. And I guess they're right. The question is, how good of a plot?
Now, I am by no means an expert at plot, but since I feel it is a major weakness for me, I've been studying it quite a bit. I've read a few books about it to help me understand it better. Now, whenever I read a book, watch a TV show, or a movie, I analyze the plot and look for what worked and what didn't work.
The thing I've been focusing on in my own writing is making sure things happen for a reason. For instance, when I was working on my sixth draft, I realized that an important chase scene didn't have a purpose. When I asked myself, "Why are the bad guys chasing the good guys?" The answer came back, "Because I want an action scene here." Wrong answer. I had to go back into the scene and give the baddies a reason to be chasing, and now there is more suspense because of it.
In Revision and Self Editing by James Scott Bell, he mentions finding the three O's in each chapter: Objective, Obstacle, and Outcome. By keeping track of these three elements, I've also been able to improve the quality of my plot by making it more exciting.
The biggest challenge is trying to think of a plot that people can't predict, but at the same time is set up well. That's a delicate balancing act which I will not claim to have solved.