In No Real Order: (This also includes WB's "The New Batman Adventures)

1. Harvey Dent, Bruce's best friend, is turned into Two-Face.
2. Batman: The Animated Series hits the theaters with "Mask of the Phantasm."
3. Mr. Freeze debuts in "Heart of Ice."
4. Dick Grayson becomes Nightwing.
5. The show changes titles to "The Adventures of Batman & Robin."
6. The introduction of Barbara Gordon.
7. The opening credits.
8. "Legends of the Dark Knight" episode (from WB's version).
9. The long awaited debut of the Riddler (like, episode 40).
10. Every Joker episode ever!

I kind of ran out of ideas after a couple. Anybody have their own lists.