I think primarily, they'd have to show some sort of 'promise'. Now whether that 'promise' is amazingly good looks, or fantastic athletic ability (Gwenog Jones), or academic excellence (Lily Evans) or political machinations (Tom Riddle), he'd have to think that whoever it is is going to go far with whatever they choose to do.
I think they also have to show some sort of potential loyalty to Slughorn for helping them with the connections, or the ability to be manipulated into doing so if it wouldn't occur to them that the whole "you scratch my back, I'll scratch yours" thing will work for them in the future.
Just a quick note to add that though I agree with cmwinters, I think that Slughorn also looks for students who have connections and not necessarily abilities. So you could have parents that were high-ranking Ministry officials and very little talent, but Slughorn would still scoop you up in the hopes that the connection with you would help him pick up favors from your parents.
I think that Slughorn is a good example of a "good Slytherin". I think that he tends to think of himself first, yet he does not do so with the ruthlessness of his fellow Slytherins. In the books he acted in a prejudiced manner, expressing surprised that a Muggleborn could be a skilled wizard/witch, however he never seemed to do so with malicious intent.
Hope this helps,
I agree with Gwen. We have to remember that Slytherins aren't only classified as 'evil'. They are cunning, sly and intelligent. Slughorn is quite cunning and intelligent. And when he set his mind to something, he couldn't really go astray.
I think that a lot of the time Slughorn acts like the 'typical Slytherin' (when he refers to Hermione in HBP when Harry is trying to convince him to go back to Hogwarts). But really, he looks for something that makes the individual person rise up above the others. He acts like he doesn't like Muggle-borns at first because he is unsure about Harry's reaction. Because, really, Harry could've hated Muggle-borns for all we know (it is common knowledge that he hates his Muggle relatives, after all). But once Harry makes it clear that he has no prejudices against Muggle-borns, Slughorn acts himself and ends up talking about Lily and Hermione with pride and amazement, not loathing.
He wants people to think highly of him, yet he won't put himself in the spotlight.
How long do you believe that Slughorn taught before he went back into retirement? Do you think he was able to hand choose his replacement, or at least had some say in it?
Although in my fic, I had Slughorn remaining at Hogwarts for many years (because I liked using him as a character), I think in all probability he would have gone back into retirement as quickly as possible. He was old when he retired the first time, a generation ago. But I am certain, Slughorn being who he is, he would have made sure he had a hand in picking his successor.
Originally Posted by OliveOil_Med
I'm currently writing a one shot in which Slughorn has lost all control of the Slytherin House, and has even stopped admitting Muggle borns into his Newt Level class because his young Death Eater Minions don't like it. I always sort of liked Slughorn in his "Pass the sweet and sour pork" demeanor, but at least right now, I just see him as incredibly weak.
It always amazes me, though that he is able to go through life alive and well and not a Death Eater, especially with underage Death Eaters hanging around the dungeons, hoping to make a name for themselves . . . Hmmm . . .
Oh, anyway, my point was just that it never entered my mind that good ol' Slughorn wasn't a Slytherin, and this thread was a bit surprising to me!