Kingdom Hearts Final Mix features changes from the original version, including bosses present in the English version, but not in the original Japanese version. Other changes include recoloring Heartless and introducing new Heartless, new weapons, new enemies, new abilities, new cutscenes, Gummi Ship missions, new Ansem Reports, and a new secret movie.
Not a lot no, then again haven't really played it in ages. I think most of the newer content was the extra stuff added in for the western releases like extra and super bosses and such. I think the main unique content is a new difficulty level, an extra secret boss, pallet swaps and a extra ending movie craze that started with it as well.
I would say that this is less of a milking than Coded/Re:coded is. The core Kingdom Hearts fan base has been asking for this for years, and bringing the first game into the next generation might help new gamers who never owned past consoles or skipped KH1 the first time around get a chance to play the game. Even looking at the few post in this thread, it's clear this is the kind of game needed to help the series, as opposed to things that have nearly destroyed it (Dream Drop Distance).
I never played a game in the series, so I am curious about this one due to its popularity. Do I understand that it is two games in one? Or perhaps the extra Chain of Memory content is spliced into the main game?
Also, graphically is the HD release getting new textures, AA, etc., or is it simply being bumped up in resolution?