I think it's a balance between being bored and being challenged. Sometimes I just want to do something mindless that I don't have to think about much....but less than half way through I tend to grow impatient because it is so mindless, and so then I just want to be done with it.
For me the perfect project is one that is complicated enough to have to use my brain --and then feel I've accomplished something when it's finished--but not so complicated that it terrifies me or makes me want to throw it across the room. I've just been working on a quilt with pinwheels which are not difficult for most of you, probably, but have given me fits because I didn't really know how to make them work. I've learned something, having to figure out what I was doing wrong when they didn't go together as perfectly as I wanted, and that has been satisfying (and more than a little frustrating!).
Bottom line is I usually fluctuate between these two--mindlessly simple or reasonably--and doable-- challenging.