1. Desmond's flashback wasn't really a flashback. Although I'm not really sure what you would officially call it, it's a new tactic for the Lost writers. I thought it was pretty cool and definitely only relevant for Desmond. They couldn't do a similar flashback for any of the other characters as far as having them relive a part of their life with bits of memories from the future. It wouldn't work.
2. Perhaps Charlie is slightly jealous of Desmond playing the knight in shining armor? At least until he realizes he was the one being truly saved, not Claire.
3. A rare occurence in the show, Lost includes unoriginal music in tonight's episode. Well, at least this is the first time I consciously noticed it. Generally, the music on Lost are original compositions. And since the Lost writers are famous, or perhaps infamous, for including small clues and easter eggs throughout the episodes, I wonder if the songs from this episode mean anything deeper. The first one was Building a Mystery by Sarah McLachlan. The second one, Wonderwall originally by Oasis, was being played on the streets by Charlie. The last one was Make Your Own Kind of Music by Cass Elliot, originally of Mamas and Papas. I think Sarah's song we're supposed to take the surface level. Wonderwall is about figuring out where your path lies and taking it despite obstacles. But maybe this wasn't about Desmond finding his path but Charlie. Especially considering Charlie is the one Desmond's been saving not Claire and Charlie was the one singing the song. Mama Cass' song Make Your Own Kind of Music was playing the first time we met Desmond in the season premiere of the second season at the very beginning as we watched him around the hatch. (Don't be too impressed. A small amount of research turned that information up. I didn't remember it by myself.) I haven't been able to find any reference to an airing of the other two songs in previous episodes. Though that doesn't mean it didn't happen. If anyone knows, please comment. I'm interested to know if they didn't play before.