And, I have to add: One of the things that I loved so much about B/R in season 3 is that it fits within the theme of the show, besides just being hot. Even though it's a sitcom, HIMYM is essentially a romance, or the opening act of a romance, and discovering love in unexpected places is a great part of that.
You have Lily and Marshall who are the couple-in-progress, who've found love but are busy with life AFTER the 'happily ever after' begins. And we have Ted/? which will eventually be what Lily/Marshall is, but he hasn't found it yet. And then, unexpectedly... you get Robin/Barney which is a completely different kind of dynamic, with a different kind of fairy tale. Not the kids and white fence and domestic bliss, but something else... a kind of modern, grown up love story for the people who don't want Ted's ideal fairy tale, but who still deserve a romance that's worthy showing on screen as peer to the "traditional" kind that the other two couples have or will have.
More HIMYM discussing because I'm in a mood right now
You have Lily and Marshall who are the couple-in-progress, who've found love but are busy with life AFTER the 'happily ever after' begins. And we have Ted/? which will eventually be what Lily/Marshall is, but he hasn't found it yet. And then, unexpectedly... you get Robin/Barney which is a completely different kind of dynamic, with a different kind of fairy tale. Not the kids and white fence and domestic bliss, but something else... a kind of modern, grown up love story for the people who don't want Ted's ideal fairy tale, but who still deserve a romance that's worthy showing on screen as peer to the "traditional" kind that the other two couples have or will have.