The projects I consider to be UFO's rather than slow moving WIPs are mostly because I'm not sure what to do next. One of the first quilts I started I really like the fabric and design, and want to finish it, but its from before I learnt to rotary cut and to be careful about sizes, so I think I need to unpick a lot and sort out the sizing.
I find that quilts intended as a present for a someone tend to stay on track better, but then they're also the ones that crop up and help to make something else into a UFO.