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Hm, I think that IX definitely had a lot more seriousness to it than FF V.
The black mages and Vivi
Kingdom of Burmecia
Cleyra
Mist
Garnet's sadness over the corruption and death of her mother
Iifa Tree
Garnet's kidnapping
Eiko's kidnapping
Terra
Mist spreading like wildfire throughout Gaia after Terra is destroyed by Kuja
Memoria
There are others but I can't really think of them all right now. In FF V you have a couple "pets" that die, the death of the Four Warriors of Dawn, and some kingdoms that get sucked into the void at the very end.
I'd hardly say that there's any comparison. Obviously FF V has some serious elements to it, but I don't see how FF IX is even less serious than that.
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X-2 is so light and fun. It was cute and I want to play again.