Confession: I never went on Horizons. Will I be shot for this? :question:
I'd have to say the original Living Seas is what I miss most from Epcot, where admittingly I spend the least amount of time (I usually only hit that and MK, possible MGM for TOT). Being a diver, and having a longstanding love of undersea life and exploration, as well as having personal links with many of the companies featured in the pavillion, it was just a perfect experience for me. I'll have to wait and see how Nemo affects this.