I think the greatest aspect of this episode was the mood. It felt incredibly dark. And there was an great sense of mystery behind the broadcast. I think what acted good as counter-balance to this dark mood was the animation used in Walter's trips. My only complaint is that while the Monty Python one was fantastic, it seems misplaced and felt like a mood breaker rather than a scene used to balance the mood. Maybe if it had eased into the scene, rather than abruptly throw us into it, it would would have worked better.
Fringe in its final episodes is reaching into it's past. It's really quite fantastic. When the fifth season began airing I was thinking that we'd never find out what happened to Walter's lab assistant. I cheered with Joy when she appeared on screen. A great performance by her too! Joining her was a younger Nina. Blair Brown was just as amazing here as always. I like seeing this human side to Nina, as in season 1 her character was presented very coldly, but she really isn't and we see this in her interactions with Walter. Other than that there was the reveal of the young Observer. And also, for the second time this season, obvious references to the Fringe symbols such as the Frog and the Seahorse. In the episode "Through the Looking Glass and What Walter Found There" Walter even pointed out the symbols. This makes me wonder if more will be revealed about the symbols which have appeared throughout the series. (If you remember back in the episode "The Bishop Revival" the Seahorse was a signature that Walter's father used.
Now to the visuals of the episode. The projector at the end, Walter watching the team on the television, the dark opening at the start as the radio received the broadcast (Great use of sound in this scene by the way). But my favourite is most definitely the scene where Walter stands in the lab looking on as Nina reaches out to stop Walter crossing to the parallel universe and the everything is completely frozen except for the mist. It was so powerful visually. This music in this scene was so moving also.
As for progression of characters we see Peter and Olivia overcoming past events. But really this was Walter's story. He's a man haunted by his past, desperately trying to save the future. He looked in the book when he new he shouldn't have. Has he really changed at all? His hallucination of Carla Warren doesn't seem to think so. I love the character of Walter. And I love John Noble's performance.
Contrary to what seems to be popular opinion on the episode, this is potentially my favourite episode of the season. I thought it was great and necessary addition to the show that pulled from the show's past, answering the question of the lab assistant fire, bringing to light the man Walter once was and potentially still is, and the discovery of the young observer. There is a great sense of cohesion created through this episode which must be incredibly difficult to do for a show that has constantly reinvented itself.