daisy_knotwise and I have been kicking Deathly Hallows about now that she's finished it. In a completely spoiler-free fashion, I've arrived at an observation about Slytherin House, catalyzed by some additional discussion over in allisona's journal:Gryffindor and Slytherin are both intended to produce leaders. It's just that they produce different kinds of leaders, both useful in the right circumstance. The leaders from Gryffindor are the boss that you love to work for, while the leaders from Slytherin are the boss that you hate to work for.
Sometimes you need Winston Churchill, but other times you need George Patton to get the job done.