Even in a plot-driven story, characters are key. I've found that one of the easiest things to do is to get so wrapped up in my protagonist, that I fail to create an adequately drawn antagonist. It's often unclear what exactly the antagonist wants and what they're willing to do to get it. And without that, the story withers and dies somewhere in the middle and has no clear ending.
For my current project, I've already sketched out both characters, but need to spend more time. Again, the antagonist needs clearer goals and motivations. I also need to make sure that the cast of secondary characters is well-rounded, since the plot will be complicated and they will add much to the texture of the story.
The good news is that I'm actually working on the project, instead of just thinking about it.