It's Four-fold and more, first of all there's the spectacle, over and over again, always surprising, always different. Secondly, there's the panels, the author tracks, the comics/ graphic novel tracks, Brit track, Dark Fantasy Track, Alternate History/Steampunk track, and so much more I didn't investigate personally, then the vendor and art rooms, and last but certainly near the top of the list - those serendipitous meetings and moments.
For me I had two surprises: - The Brit track was one of the highs of my con, seeing all the DR. Who's and Tardis' (how do I make this plural?) walking around or standing in full life size 3-D reality in the Brit Track room, was a highlight. (Here's the life size Tardis in the Parade, photo by Tracy V. Wilson).
But then to see panels on all my favorite BBC shows, Being Human, Torchwood, Dr. Who and even the brief but amazing, Primeval. Unfortunately I didn't make all of these panels due to getting lost or attending other panels, but to know they available was a gift to save for next year.
Also, I discovered how fascinating the academic comics track was fascinating. Riveting erudite presentations and vibrant question and answer sections with audiences highly invested in this milieu Even for me an infrequent comics collector (I own a signed faves: The Crow, Sandman, The Girl Who Would Be Death, a number of items by Neil Gaiman and Dave McKean) but I'm not a collector or even an avid reader.