So, everyone's big complaint about the first game is the structure of the second half of the game. You spend the first half of the game making your way around the world trying to reactivate the four crystals. At that point, the game introduces you having to travel to alternate worlds where you basically have to do the same thing in 4 more worlds in order to get to the real ending. And while there's 4 times as much of the game here, it's really only the 2nd half of the game because you have the airship at that point, so it goes much faster.
The problem of course, is that these alternate worlds, are all the same as the first world. It's got all the same towns, same terrain, same dungeons. The monsters are all stronger, but in true FF fashion, they're just palette swaps of earlier monsters. The only thing that's different, is the characters. Each world has all the same characters, but they have completely different personalities. A character that was a villainous asshole in one world might be a noble hero in the next one. If you're a fan of superhero comics, it's basically the same as an elseworlds story.
Now, don't get me wrong, I can totally understand why people would hate that, and it's definitely lazy. But I didn't have a huge problem with it. It was fun getting to interact with all those character(most of which you kill in the first world) in all these different ways.