I never felt that Adam takes anything away from the specialness of Sam and Dean's brotherly bond, instead it adds to it by showing us new dimensions to it, challenging their positions in a way no random other character ever could.

Yes, totally. That's why I was always open to the thought of a half-brother as everyone was fretting about it when it was only a spoiler. I think SPN's view of family is quite subtle: blood definitely matters (Sam and Dean care more about Adam than they would about a random kid because he's blood), but it's not enough to make people family. And on the other hand, non-related people can become close enough to be family. Sam and Dean are special to each other because they're blood and because they have a long, long history together, have overcome a lot of obstacles together, have shared a lot of pains and joys.