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Great question! There's an essay called "Is Ambition a Virtue? Why Slytherin Belongs at Hogwarts" in a book called "Harry Potter and Philosophy: If Aristotle Ran Hogwarts," edited by David Baggett and Shawn E. Klein. I reviewed it in my essay on Snape and the Slytherins:
http://asylums.insanejournal.com/snaped
One can be ambitious for beneficial reasons--for example, someone who wants to become Minister of Magic to change the laws that discriminate against non-humans. One can even be ambitious for relatively selfish reasons, and still not be evil, such as the desire to become a world-class Quidditch player or a famous musician. I wonder if any of the Weird Sisters were Slytherins? ;-)
Cunning, another Slytherin quality mentioned in the Sorting Hat's song, has a negative connotation, as it seems to imply sneakiness and deception, but I think it can be a good quality as well. After all, Snape needs a great deal of cunning to pull off his role as a double agent. The Gryffindors, by contrast, tend to be brash and wear their hearts on their sleeves. Even shy Neville openly defies the Carrows in DH, rather than quietly organizing his resistance in secret--which would have been safer, and perhaps more effective. I doubt that James or Harry could have convinced Voldemort that they were loyal Death Eaters, but Snape did, and his role was vital in helping to protect Harry and win the war.
I also think, as others have mentioned, that a sense of self-preservation is a Slytherin quality--one generally has to be alive to achieve one's ambitions, after all! Even JKR admitted that this was a good thing during her PotterCast interview, when in a bit of canon revisionism, she claimed that the Slytherins returned to fight at Hogwarts with Slughorn, after going to get reinforcements first. Never mind that the scene is not actually in the books. *rolls eyes* At least she admitted that the Slytherins weren't evil.
(I discussed the interview in this esssay: http://community.livejournal.com/snaped
I do think that under certain circumstances, the Slytherins would be willing to risk their lives, but most of them would require more personal reasons than just "saving the world," particularly for the younger ones--the lives of their family and friends being threatened, for example. As Bohemian Spirit mentions above, this is probably related to the self-preservation instinct. For most people, that sense of "self" expands to include their loved ones, and with maturity, it might continue to expand to the point where it does indeed expand to include humanity in general. Snape betrays Voldemort solely for Lily's sake, and then protected Harry for her sake. But at some point, he must have agreed with Dumbledore that defeating Voldemort was more important than protecting a single life, or he never would have let Harry sacrifice himself. He could, after all, have attempted to Obliviate Harry and send him off to another country where he might be safe, as Hermione did with her parents. Instead, he passed on his memories and Dumbledore's instructions to Harry, even though--unlike Dumbledore--he didn't seem to have any hope that Harry might survive his sacrifice.
The essay also points out that Harry himself shows ambition--he was nearly Sorted into Slytherin--by wanting to prove himself and live up to the memory of his parents. The author also mentions some real-life examples of admirable ambition: "Was Gandhi not ambitious when he undertook to gain independence for India? Was King not ambitious when he organized and led the fight for racial equality in the 1960s?"
The author argues that ambition should be thought of as a virtue because it is the desire to excel; people achieve great things because they want to excel, whether it is in the field of athletics, science, or business. It is only when someone has an excess or deficiency of ambition that a problem arises.
http://asylums.insanejournal.com/snaped
One can be ambitious for beneficial reasons--for example, someone who wants to become Minister of Magic to change the laws that discriminate against non-humans. One can even be ambitious for relatively selfish reasons, and still not be evil, such as the desire to become a world-class Quidditch player or a famous musician. I wonder if any of the Weird Sisters were Slytherins? ;-)
Cunning, another Slytherin quality mentioned in the Sorting Hat's song, has a negative connotation, as it seems to imply sneakiness and deception, but I think it can be a good quality as well. After all, Snape needs a great deal of cunning to pull off his role as a double agent. The Gryffindors, by contrast, tend to be brash and wear their hearts on their sleeves. Even shy Neville openly defies the Carrows in DH, rather than quietly organizing his resistance in secret--which would have been safer, and perhaps more effective. I doubt that James or Harry could have convinced Voldemort that they were loyal Death Eaters, but Snape did, and his role was vital in helping to protect Harry and win the war.
I also think, as others have mentioned, that a sense of self-preservation is a Slytherin quality--one generally has to be alive to achieve one's ambitions, after all! Even JKR admitted that this was a good thing during her PotterCast interview, when in a bit of canon revisionism, she claimed that the Slytherins returned to fight at Hogwarts with Slughorn, after going to get reinforcements first. Never mind that the scene is not actually in the books. *rolls eyes* At least she admitted that the Slytherins weren't evil.
(I discussed the interview in this esssay: http://community.livejournal.com/snaped
I do think that under certain circumstances, the Slytherins would be willing to risk their lives, but most of them would require more personal reasons than just "saving the world," particularly for the younger ones--the lives of their family and friends being threatened, for example. As Bohemian Spirit mentions above, this is probably related to the self-preservation instinct. For most people, that sense of "self" expands to include their loved ones, and with maturity, it might continue to expand to the point where it does indeed expand to include humanity in general. Snape betrays Voldemort solely for Lily's sake, and then protected Harry for her sake. But at some point, he must have agreed with Dumbledore that defeating Voldemort was more important than protecting a single life, or he never would have let Harry sacrifice himself. He could, after all, have attempted to Obliviate Harry and send him off to another country where he might be safe, as Hermione did with her parents. Instead, he passed on his memories and Dumbledore's instructions to Harry, even though--unlike Dumbledore--he didn't seem to have any hope that Harry might survive his sacrifice.