Aside from the GS ball, Johto promised tons of things and failed to deliver them:

- Ash promised Totodile it would evolve in "Ring Masters" when it showed it that Feraligatr, and it never happened. He even let Totodile watch the sumo match, and then Totodile doesn't go through any development the rest of the arc.

- In one of the Larvitar eps, I can't remember which, Ash contemplates evolving Cyndaquil to make it grow stronger, since at that point Ash finally got it in his skull that evolved Pokemon tend to get stronger. However, by the end of the ep Ash abandons the idea and doesn't touch on it again.

- Brock gets a Heavy ball from Kurt that he never uses. What was the point of Brock getting so hyped up about a Heavy ball and the writers not using it?

- Togepi's metronome powers are never discovered by Misty, which is bizarre, so Togepi's plotline essentially is badly handled. AG sort of fixes this by the writers finally evolving Togepi and moving on.

- In the Gligarman episode, Gligarman gives Brock a whistle for them to call him when they get in trouble. This is never used again.

- Casey started off seeming like she would be a secondary rival this arc after Gary, but as time goes on she just fizzles out. There's a reason most people tend to forget Ash actually did have a female rival at one point, she was just so badly handled that people tend to forget about her.

- In the Johto league, we discover via flashbacks that Ash and Gary had a childhood together. Yet in the first ep in Kanto, Ash and Gary act as if they never met each other. Talk about a retcon here.

- In the Todd arc when he wants to take a picture of Articuno, Ash acts as if he never saw one before and doesn't think it exists. This is a plothole because as the show goes on, Ash remembers the legendary Pokemon he sees in movies when they re-appear in episodes.

I'm pretty sure that's it, unless I'm forgetting a few things.