I think this game's replay value is much greater than it was in the first game.
In the first game, the replay value was pretty much getting all Pokémon (only 257) and then replaying levels countless times to find the shinies you want.
In this game, there are 646 Pokémon, and the different forms get registered in your Dex as well. And although shinies can't be found in the game, there is a great replacement, rare abilities. The abilities greatly improve the stats, and it's thereby possible to get a Pokémon with an extremely high speed or attack.
Also, you can now actually choose which moves your Pokémon get, both by the specific move vending machines and by having Pokémon teach other Pokémon moves.
And with the world ranks, every Pokémon can be found at a high level. They do cost a lot, though, so you'll have to get a lot of money to get to rank 10. And when you have a little team of high-leveled Pokémon with rare abilities, you can use them to challenge the EX levels. You can even try to beat the EX-6 Battle Royale and/or Team Battle with a specific species of Pokémon, or maybe even beat them with every Pokémon there is.
Seriously, if you liked collecting shinies in the original, I don't see why you wouldn't like this.
I also have a question about StreetPass.
What determines the level of the Pokémon you get by StreetPass?