I especially loved how, in Utopia, right beforeSpoiler:you started hearing sound clips from the old series. Thought that was an incredibly nice touch, and after 3 years of not really seeing or hearing anything before 2005, a genuinely fan pleasing moment.
Btw, your sig/avatar keeps reminding me: I must get back to Fringe! I've only watched the first couple of eps, about a few months back.