Apart from Silph Co. (which took me way too long), the most annoying part for me is there really wasn't all that much to do after you caught Mewtwo. If a game from the current gen lacked a post-game, everyone would be outraged, but even as a kid I was frustrated there was nothing left to do in Yellow, especially since I loved the game so much (no one to battle but the Elite Four again, no other places to travel to, etc.).