I too offer up another echo into the chamber and say "Costumes", as possibly my favorite thing about CoX. Dear Zod, the sheer number and types of options were staggering overall.
Sure, there were obvious things that should have been in there (Like sweatshirts, hooded and otherwise), but I have no doubt we would have seen a lot of our wish-fulfillment in the area of costumes granted, because over the years we saw a lot of requests realized. One of my big wishes would have been more Egyptian-styled pieces, and at the rate we were getting new sets added I'm sure it would have happened eventually.
As it is, what WAS there was just gorgeous. Some of the new stuff (Like the phoenix wings, the ascension set, bio-armor, Imperial Defense, etc) just blew me away with its creativity and execution.
Also, I have to give a special shout-out to my favorite travel-power, Super-Jump. It was fun and thrilling and fast, and I adored it.