I didn't like Johto at at all. I only liked the day/night system when it came to the Gen IV games because I felt it was too erratic in G/S/C, I never played far enough in Gold to experience the roaming legendaries, I never discovered that you could breed pokemon (I did get the Togepi egg, but I never discovered that I could breed pokemon), I found most of Johto through Azelea(?) to Goldenrod and the fourth gym town to the sixth (can't remember their names) uninteresting
and I certainly never discovered that you could travel back to Kanto (which, after playing through Heart Gold, I agree with LTM with in being lazily shoehorned into the game).
Even the pokemon didn't help much. Half of the new pokemon didn't appeal to me much and some were downright infuriating to fight during the course of the main game (ie. Whitney's Miltank). The only redeeming factors I got from G/S/C was the Eevee evolutions and the mascot legendaries (I think I stopped after catching Oh-Ho).