As far as Pokémon post-games go it's very much significant, and there's good reason why it's remembered so fondly. You, the new Indigo League champ, get to travel to a whole new (or rather, the old) region to challenge the Gym Leaders there. It also ties up the story of Blue and Silver, alongside some other tidbits such as Cinnabar Island being blown to bits and Janine replacing Koga. It's merely a watered-down Kanto, but nothing has come really close since. I don't remember the optional areas in Johto being anything near as impressive.