The Kanto post-game brought lots of new Pokemon to the table, like Snorlax and many others that were simply uncatchable in Johto. And many people, including me, liked getting to fight the good old Kanto gym leaders again, after experiencing them for the first time in Gen 1 or 3. Kanto was not really in the best shape, but it is only a Game Boy Color game and GF could only fit so much on the cartridge. I do wish they included the Safari Zone, though (There's even an unused map, albeit very small, for it in the game)...
And as for Mt. Silver, there was the implication that something very, very powerful was at the top of that mountain. I mean, Prof. Oak won't even let you step foot in it until you have 16 badges, and he said there were crazy powerful Pokemon in there. It's like Generation II's version of Cerulean Cave. As soon as Oak lets you in there, you know something's waiting for you at the top. Almost every dungeon in GSC has something at the end to be waiting for (Sprout Tower has HM05, Mt. Mortar has Tyrogue, etc.). And when you finally reach the top, after hours of sweat and tears navigating Mt. Silver, you feel an adrenaline rush as you talk to the mysterious character at the top of the mountain and battle the protagonist from the previous games, who is the toughest, and last, boss in the game. While I do agree that most trainers in Kanto were total pushovers, I personally liked the nostalgia trip.
And about Unova: As I said earlier, I haven't played Black/White yet, but from what I have heard, BW2 has more postgame. I wouldn't know, though. BW2 had the PWT, which I really enjoyed also, battling all of the previous Gym leaders in a semi-competitive setting. BW2 also had a whole chunk of Unova to explore, although the only things that mattered there were Cresselia, the stone to summon Heatran, and the Day Care. There's also Icirrus City and all the Dragonspiral Tower stuff with Zekrom/Reshiram, and a bunch of other legendaries. But that's my point: Pretty much all the postgame in Unova (or at least BW2, except PWT and Black City/White Forest) revolves around legendaries, which really, when you think about it, defeats the purpose of being a legendary Pokemon in the first place. There's just so many of them in-game that they sort of lose their value. I feel that way about ORAS as well, what with having twenty-something legendaries to find across Hoenn.
So in summary, I think both Gen II and Gen V have a lot of postgame content, but I liked Gen II's better personally because it didn't all revolve around a bunch of unnecessary legendaries.