Well, I hardly remember playing my Yellow version. I can distinctly remember playing the Crystal version though. It's nostalgia for me also, haha.
Any game that I can remember playing brings back good memories. And G/S/C does just that. It was a step up from R/B/Y. More was introduced to the games. The later games just took these things and expanded on it. I loved the plot. Not the whole Team rocket thing, they were boring and tedious and should have stayed in R/B/Y where they belonged, but the legends surrounding Ho-Oh, Lugia, and the legendary beasts really interested me. The Advanced generation and the 3-D games don't do that.
Also the gyms are so memorable. I doubt anyone here has not heard about Whitney and that godforsaken Miltank of hers. And Chuck being a blatant ripoff of Chuck Norris. xD haha.
Also the game introduced shiny pokemon too. Kudos to the red gyarados! Because I think he was the only shiny pokemon anyone basically knew about unless they were old enough to use the internet and look around for any type of article that said you could get other variations of the same pokemon.
Not to mention when listening to the music it sorta brings a tear to my eye because it makes me think of how awesome it was to be a kid again.
I really think the games are nostalgic due to more people playing them because it was way more popular than the first gen and the first generation players were old enough to understand the game as a whole.
