i think there should be no down-vote - because i very rarely notice that someone comments that doesn't make no sense at all - also if people who get up-voted will automatically gain more value anyway...

so for example if i make a comment that doesn't make sense then nobody will up-vote me (and nobody can down-vote me either as there is no option to do that either) and i will end-up having lowest score (which is 1) compare to others. also down-vote de-motivates more than no-vote... so why de-motivate someone.

i hope i explained in simplistic way...