I don't really like the idea of seeing who negative repped a post. Like, I'm a pretty sensitive guy, and it be would kind of deconstructive to have "PERSON A, PERSON B AND PERSON C ALL HATE YOUR POST."
What if nobody wants to make themselves look like an ass and negative rep people? Then there would no purpose of negative rep. Actually a negative rep war could spark e.g. One guy likes to negative rep people so everyone feels they need to negative rep him.
I'm in favour of the positive only system.