The top 3:
(3) Durability, particularly in the melee classes. My Tankers were stone walls (sometimes literally) taking everything that came at them, my Brutes were wrecking balls laughing as return attacks bounced off.
(2) Masterminds. So many pets. So much fine control. The Pet controls in other games are a pale imitation, if there are any at all.
(1) Any incredibly cool character I thought of in my head always translated perfectly to an incredibly cool character in the game itself. I loved every character I made, on all levels - costume, powerset, effectiveness, animations, etc. No other game has ever done that.
I loved many other things, like flying (of course!), and the sheer usefulness of Buff/Debuff/Control sets (a rarity in RPGs, let alone MMORPGs), and how quickly updates were coming (there was always something new and exciting on the horizon!), and the world and the stories and the oh god I still miss it horribly.