Personally I think they have always been sailing a bit close to the wind with the Mac/Caroline relationship, in that both characters teeter on the verge of being so perfect in every way that you are tempted to dislike them. Mac's fatal illness and his bravery was toying boldly with goody-two-shoes-iosity. I think Green Wing just manages to stay on the right side of the fence, partly because while Tamsin and Julian are both attractive folks, neither is conventionally good looking in that boy bad/starlet sense that is so dreary.
I think Andrew is right (comment to previous post) to say that the episode 'manages to bring Guy across as an amazing bastard and make me sympathetic towards him'. I was afraid they might make him a reformed character, but instead he was if anything worse than usual, while struggling (along with Boyce) with a wakening moral conscience. I thought they might have ended up with a three-in-a-bed solution to that particular triangle. No I didn't really, but it would have been a blast.
Caroline was, however, forced to protest that she didn't want to get into Dame Judi Dench's pants. I bet she does really.
The office girls went a bit feral. Karen lost part of her body and found a personality. Harriet hasn't had her baby yet. Katie Lyons continues to be particularly scrumptious as Naughty Rachel (there's another non-starlet but very good looking lass). These are a great bunch, but I thought their story was least well developed of the lot. I suppose there wasn't time.
My favourite characters in Green Wing, though, are Boyce/Joanna/Statham - last seen failing (sob) to get it on at the end of Series 2. I hardly know where to start with this strand, because I am tempted to just recount all the things that happened. First they.. and then they... did you see the bit where they... One of my favourite scenes was when Boyce silently smells and kisses Statham's lecture-pointer, in a scene that (IIRC) echoes Bob Peck fondling Joanne Whalley's vibrator in one of the greatest TV shows ever made - Edge of Darkness. Talk about iconic.
Boyce and Statham never meet again, though they obviously missed each other, and had one broken phone call. All three characters, deeply flawed as they are, achieved some kind of redemption.