Cash Cow Franchise: Within its first 10 years, Spongebob made over 8 BILLION DOLLARS in revenue. No doubt that it's still making more.
Cross-Dressing Voices: Patrick and Mermaid Man are voiced by Ikuko Tani (an old lady) in Japanese; Plankton, however, is voiced by Chie Matsuraa, a young woman who doesn't even try to match Plankton's English voice.
Executive Meddling: It has saved the show from two series finales. The movie was intended to be a Grand Finale for the series, but the possibility was later pushed to the "Spongebob's Last Stand" special. The show was continued since it was very close to dethroning Rugrats as the longest running Nicktoon. It eventually did bypass the series when it aired episode 173.
The Other Darrin: Man Ray's first appearance in "Mermaid Man and Barnacle Boy V" was voiced by Guy Siner and John Rhys-Davies. Man Ray's subsequent appearances have been voiced by Bob Joles.
The voices of the background fish change a lot.
In the Spongebob video game Battle For Bikini Bottom, Mr. Krabs was voiced by Joe White instead of Clancy Brown for unknown reasons.
Joe White also voiced Mermaid Man in the video games, and has replaced Ernest Borgnine as Mermaid Man on the show following Borgnine's death.
Recycled Script: A few old episodes of the show seem to have recycled plots from other Nicktoons. "Toy Store of Doom", for example, has essentially the same plot as the Rugrats episode "Toy Palace" (they get locked in a closed toy store and are afraid the toys will attack them), while "Banned in Bikini Bottom" (Krabby Patties are outlawed and Mr. Krabs starts selling them at SpongeBob's house secretly) is similar to the CatDog episode "Just Say CatDog Sent You".
"Picture Day" has a recycled script from an episode of a Disney show: the Recess episode "One Stayed Clean". "Big Pink Loser" is also extremely similar to "Copycat Kid".
"Sailor Mouth" is very similar to "Bleeped", but airs much more frequently.
"Squid Defense" is very similar to the Hey Arnold! episode "Mugged"
Word of God: Stephen Hillenburg has made it perfectly clear that Spongebob does not feel sexual attraction. He also adds that Spongebob has a chaste crush on Sandy.
In terms of the series itself Word of God also states that in spite of the series continuing to this day; the original team members and the Hillenburg himself say that the first theatrical movie is the end of Spongebob's story.
As the second movie is starting to be filmed in Savannah, it's anybody's guess as to where the story stands now.