I came across a great talk today by Matt Gilgenbach of 24 Carat Games, which gave me a lot to think about. As a lone developer with a limited budget (I'm also self-funding) and limited time, it's important to prioritize.
My current habit is to look at what I've accomplished so far and think, OK, in what way is it most obvious that this isn't a finished game? Whatever the answer to that question, that's what I work on next.
I still think this method is going to work if I stick to it. In fact, I have renewed confidence that it's the right way to work. But what I now think might not work is the current art style of the project.
I've been designing with a view to a semi-realistic 3D art style, but I'm going to look into dropping that for a simpler, more abstract look. Most of the graphics are still placeholders, so this won't mean much wasted work.
As Matt says, as an indie, you aren't going to compete with AAA studios in terms of graphics. I'd be better off spending that time on level design, game features, gameplay polish and the level pit.