Adaptation Sequence: Book —> Movie —> Stage musical —> Movie of musical —> (Extremely) short-lived Saturday morning cartoon show.
Cut Song: Most of "The Meek Shall Inherit", the "Somewhere That's Green" reprise, the end of "Mean Green Mother From Outer Space", and "Don't Feed The Plants".
Deleted Scene: Aside from the original ending, there's also the full "The Meek Shall Inherit" scene.
Development Gag: As he explains in a documentary about his life and music, Alan Menken's father and many of the other men in his family are dentists — something which he also began to study for but gave up in order to pursue music instead. No doubt he and Howard Ashman had fun writing the dentist's song!
Harpo Does Something Funny: Bill Murray improvised all of his lines as Arthur Denton, the masochistic dental patient. This is why the characters he interacts with seem to be ignoring what he says - the actors literally didn't know what he was going to say. To their credit, they managed to keep a straight face (at least in the takes that were used).
Keep Circulating the Tapes: For the longest time, although the song was included on the official soundtrack, footage of the original ending was only available in crude, sound-free, B&W workprint form on the DVD extras, which led multiple people to create YouTube fan edits of what it would potentially look like completed. (After the Blu-Ray release with the ACTUAL completed footage, all of those videos had to be taken down, and any new ones posted don't tend to last.)
Cut Song: Quite a few. Demos of them are available on the 2003 Broadway cast recording. The songs are...
"We'll Have Tomorrow": A duet between Audrey and Seymour, probably meant to take place shortly before Audrey gets eaten. Oddly, the line "we'll have tomorrow" is still sung in "Don't Feed the Plants". Unlike most cut songs, official sheet music exists for this song (released as part of the Hal Leonard Alan Menken Songbook).
"Bad": A Villain Song for Audrey II. It got reworked into "Mean Green Mother From Outer Space" in the movie, so the song would be eligible for a best original song nomination at the Oscars. (Which it received, but didn't win.) It was probably cut due to the severe Mood Whiplash that it would've caused after Audrey's death.
"A Little Dental Music": A song about Orin's practice. (Sources vary on who was supposed to sing it — some say the urchins, some say Seymour.)
"I Found a Hobby": A Villain Song for Orin, talking about how he discovered his sadism and became a dentist. This was presumably reworked into "Dentist!".
"The Worse He Treats Me": A song for Audrey, talking about her relationship with Orin. It's very "As Long as He Needs Me".