One of my favorite moments in those eps is when the Doctor figures out what's been going on with the nanogenes and explains it, and Jack finally realizes that this whole thing is indeed his fault. That look on his face, and his voice... Up to then, he seemed unwilling to take responsibility for his screw-up, but that's the moment you can see he *does* feel responsible--he just really hadn't thought his actions could have caused this. (Which makes his willingness to help Rose and the Doctor get into the bombsite even more remarkable.)

Also awesome and, of course, hilarious, is the infamous screwdriver/blaster one-upmanship conversation. "Well, I've got a banana, and in a pinch you could put up some shelves. Wow, the assets conversation went in a flash, didn't it?" *giggles*