I'm with Boaz, that it is indeed to be a solemn tune, about the folly of Castamere's pride in the face of Lannister determination. I don't think it should be jaunty, but it
could be.
A tune can be solemn, and played more quickly to make the meaning more intense, and the feeling slightly less gravid (think of
Ring Around the Rosie: "Ashes, ashes, we all fall down" (read: died)). Another much more recent example: take the difference between
Billy Jean as sung by Michael Jackson (
http://www.youtube.com/watch?v=Zi_XLOBDo_Y) and Chris Cornell of Soundgarden/Audioslave fame (
http://www.youtube.com/watch?v=R0uWF-37DAM). The first is a dance song, with tragic lyrics that you can't quite grasp, and the second is a soulful, deep tragedy, similar to way that the Rains of Castamere is sung in the link in the first post of this thread. I posted this example because the song is fairly well known to anyone alive for the last 35 years, or even less, and the two renditions are very, very different.
I think that Cornell sang it the way it should have been sung. Michael Jackson never had the gravity (or life experience?) to really pull the full emotions out of those lyrics. Cornell brings the full measure of pain and anguish into his voice, and you really FEEL it. Never felt it with MJ, except that I knew the song was sort of serious.
Rains of Castamere is great the way it was done.