It did away with a lot of Kanto's personality, mostly by surrounding all the environments with those same generic trees. Stuff like the rocks surrounding Pewter or the open space around Saffron really helped to sell the player on the sort of city they were supposed to be in, despite the limited resources of the Gameboy. Meanwhile, locations like Pokemon Lab were made to look a lot blander. The GBA soundfont doesn't do it any favors, either.
You could say those complaints are sort of nitpicky, but the problem is that FRLG didn't do enough to make up for it. While I do think the Sevii Islands were a neat idea (I especially like that they expand on aspects of Kanto that were touched on in the Gen 1 games, like the presence of biker gangs), there just wasn't enough to them. The quests should've been a bit more substantial and involved, and they needed a better climax than a warehouse with some trainers who are barely tougher than the trainers you fought in Victory Road.
Plus, Gen 1's battle system is unique enough to justify a playthrough of the GB titles, while FRLG are basically just another couple of Pokemon games.