For me, it was the whole "immersive" nature of being able to see Pokemon in full 3D in their own little environments, and then being able to interact with them in whatever funny way (throwing apples or pester balls at stuff was hilarious when I was like 7
). Going from a little top down, 2D sprite game to a fully 3D game in a bunch of different courses and areas, even when on-rails, was just super cool to me as a kid. And it wasn't just snapping photos, you had to figure out how to get the best pictures through trial and error, or figure out a sequence of events in order to get a Pokemon to evolve or to get a Pokemon to lure another Pokemon...Just a lot of nice little touches and stuff that took Pokemon from being a simple RPG to more of an "actual world" kind of thing.
But I can understand why someone might not like it, I think if you never played it as a kid/didn't really care about Pokemon all that much beyond enjoying the "RPG" aspects it'd probably not be that interesting of a game.