of course he's shallow. its the cliche thats his character. hes very pompous, and likes to collect student trophies remember? what surprises me...is that very few people know about horcruxes. very few, unless you're dumbledore or a death eater. so how does someone like slughorn come about with knowledge such as that? and if he knows about horcruxes, couldnt he tell that that's what was on dumbledore's hand? the next book's going to divulge a little more into slughorn's past i think. hopefully the condition of dumbledore's hand will also be explained
also voldemort wanted him, most likely because of his knowledge of horcruxes, which again proove that, as lady slytherin said, very few people knows of horcruxes, and slughorn seems to be the only "good" person left with much knowledge of them. (IMO snape is still good and will help with the horcruxes too, but never mind)