The top most features I use even during casual shooting is zebra and live histogram. Zebra simply highlights under/overexposed areas, that fall beyond camera light range. Histogram helps balance lights level in shadows, middle tones and bright areas. Those two save me many experimental, "light probing" shoots until I get expected result. It's very easy for use feature, even for non experienced photographer.
I don't mention other, less used features that can really expand photographing experience, if you really get into it.
Next time I am installing CHDK on all of my SD cards to not miss opportunities when I get my camera.