Well, I doubt they changed much in the 3D models and graphic engine departments. I played the game when it first launched and was shocked at how many art assets where taken directly out of Final Fantasy XI. Mind you: they looked better thanks to a better engine with better shaders and the like. Character models where brand new but nearly the same design as the previous game.
The problem with the game when it launched were not graphics (although hear that graphics did cause it to launch for PC only at first,) but gameplay issues.
It was touted to be casual friendly, that "we learned from WoW's success", but the final game was even grindier than Final Fantasy XI and Classic Everquest 1 put together. Missions consisted of traveling like crazy to go kill a handful of critters in an open area and return... then be limited to X number of quests per day... combat was... slooooooow. And the crafting system... oh lord, at launch it required you to master every single trade skill to get a single sword crafted, because they attempted to interlock crafting in a way that every skill would be required to... well... trade with each other.
Finally the refusal to implement an auction house and stick everyone into afk player kiosks.
At the end of the day, FFXI felt way more of a casual game.
As I mentioned before, the reception was so bad they made the game free for a year or so, think they fired the lead designer, started working on this new "incarnation" and decided to put the PS3 version on hold until they managed to come up with a GOOD MMO.
I understand the goal with Rebirth is precisely to change all that 180, but have never been clear (to me) on how they will do this. My fear is that during that time window they may had listened too much to the players that stuck around, those players likely enjoyed the grindfest.
I'm a BIT hopeful, though. I liked the feel of the world in FFXI and would love if FFXIV became a casual version I was able to tolerate, because after playing CoH, I was unable to ever again enjoy FFXI.