Johnny Novgorod said:
I really don't get all the Slyth fans, considering every Slyth person in both books and movies is portrayed as either being a bully jerk or an evil jerk with no redeeming qualities. Not the house of cool, magnificent bastard loners everybody thinks it is.
I think you're missing the bigger picture here.
Yes, the movies portray Slytherins as the evil jerks with no redeeming qualities? Why? Because it gives a very us vs. them attitude. It's an easy banner to lump any and all villains under for the sake of easy story telling. Is it something that annoys me in the books, yes, because it's too damn easy. There is hardly ever a circumstance in real life where people under one banner are all assholes like that out to destroy the world, and be ten shades of ineffectual asshole.
You look a little bit past that, and what do you find. Well, this:
Slytherin is one of the four Houses at Hogwarts School of Witchcraft and Wizardry, and is traditionally home to students who exhibit such traits as cunning, resourcefulness, and ambition.
That's lifted right from the Harry Potter Wiki. A lot of those traits are ones that many people can identify with. You look further at what other traits are normally attributed to them and find this.
◾Traditionalism
◾Resourcefulness
◾Cunning
◾Ambition
◾Leadership qualities
◾Self-Preservation
◾Determination
◾Intelligence
◾Fraternity
◾Power
And again you find a number of traits that many people can identify with and get behind.
Besides that, you start to look into the books, and you can find that some of the more villainous characters weren't all from Slytherin, but from other Houses as well. Peter Pettigrew for instance was from Gryffindor, and Quirinus Quirrell was a member of Ravenclaw. On top of that, one of the more dubious, though in the end more heroic characters, Severus Snape was also a member of Slytherin. Not to mention the fact that the "dunce" Slughorn that you had mentioned did regret his actions that he did, and in fact wound up fighting against the Death Eaters at the Battle of Hogwarts, and wound up not being ineffective in it, and dispatched a numerous combatants, despite being disarmed in the middle of combat and yet being able to recover his wand and continue fighting after.
In the end, what many people do is look past the most blatant aspects that are crammed into your face, and see what is most valued by the House. A number of the traits listed prior are ones people can respect and get behind, despite not agreeing with all. Because that's what everyone else is looking at here. They're looking at what characteristics are in line with the Houses, and seeing which are the ones that fit their ideals best.
Personally, it's a toss up for me. I've taken a few quizzes, and wound up with my results being mixed mostly between Slytherin and Gryffindor. Hell, the official Pottermore test put me in red and gold and I'm still uncertain of whether that'd be where I'd fit in best or not.