What are the differences between Normal edition and the GotY (Game of the Year) edition?

Some that I know of:

• New achievements
• Steam Cloud support for those on Steam
• The dancer zombie no longer looks like Michael Jackson

Blurb when the GOTY edition was released on Steam:

Plants vs. Zombies: GOTY Edition is now available with updated and expanded content including 20 new Steam Achievements, Steam Cloud support, and the interactive Zombatar™. Now with SteamPlay, Plants vs. Zombies can be played on either Mac or PC!

New info that other answers have not included:

• Now playable on Mac if bought through steam. I presume this was not possible before?

Also note, I purchased the normal version of the game on Steam, and I believe I received a free upgrade to the GOTY edition when it came out. Steam appears to only offer the GOTY edition now, since I can't find the old version in the steam store. Good-bye Michael Jackson Zombie. ;(