I'm going to voice an unpopular opinion here (or at least unpopular with me but I came up with it so here we go).
We could try a house rule that units that are not fully painted all have the Devastated status. It's a simple elegant solution to the problem. It's also a very bad solution as a unit could be seriously hampered for months on end while still paying full points to field it. While it would be nice for a unit to become more powerful when it's fully painted this doesn't seem like the best solution to the problem.
A more refined version of this idea would be if the painted portion of the unit is below minimum model count (not preferred model count as that's even more figures). That could still become a long term problem. I've toyed with keeping units of the table until they're near minimum model count so they the movement trays are not insultingly destitute. I kept the crossbowmen mostly a surprise (and there's at least one more surprise on the way) so that you chaps can be pleasantly surprised when I reveal a unit with a significant amount of paint on it.
A better solution would be to encourage and reward painting rather than to penalise it's absence. I'm not sure what manner of rewards would be suitable that would not affect the balance of gameplay too greatly 'though.
Nice idea but there are already 5 point magic items that do that. Perhaps they get to do it in their first game as a fully painted unit but not after that. A little celebration that they're finally done.
How about unpainted units restrict unlocks so they don't unlock as much? Now there's a soft penalty limit that won't directly affect gameplay but will affect list building flexibility?
The only problem with that idea is that it will lead to even more unpainted units due to the fancy stuff not being unlocked yet.
interesting idea - so you best get one unit painted to a standard that is still to be agreed upon. Will that include basing effects or will goblin green count?
Well on Monday I should be fielding two or three fully painted units and one that's 65% of the way there (with some paint on the other 7 figs). I don't consider a miniature to be finished until the base is done. This makes me hate doing bases even more because I often want to do them before I'm finished just to get things over with.