Having a character of one main class and a secondary class that could be switched at any time between any of the 9 classes.
8-slot skillbar with one heroic skill that could only come from your main class.
400+ total skills in the game.
Plenty of room for you to make your own homebrew builds, and some classic builds that were outside the box:
The assassin that used a staff (assacaster), the ranger that used necro skills to touch people to death (touch ranger), and the 55 monk, which had almost no hp but so much healing it was hard to kill.
It will always be my MMORPG because of the character design.