12's combat is one of my favorite in any rpg ever, and I really liked the way the IZJS broke up the omni-class stuff. The gambit system was a natural progression of characters who always auto-attack; characters who always cast spells; characters who always do special moves etc etc. However, I will say it directly contributed to the gameplay style that was in FF-13 with the paradigms. Instead of programming the AI via gambits, you just chose a mode for the AI and it would auto-use abilities that players would manually select anyway.
12's big problem wasn't the gameplay or the graphics or the art style or whatever. It was entirely related to the fact that the "main" character of the story isn't introduced until a couple of hours in, and the main antagonist for the entire game isn't really even a bad guy. Not to mention the alien-god monsters controlling the world, which is then never referenced again. The writers (especially since the world is apparently the same as FFT and a few other games) tried to capture the political intrigue and gripping story of Final Fantasy Tactics, but towards the end it just sort of meanders away from that and the ending isn't fulfilling at all. That, and they had to shoe-horn in Vaan and Penelo, two easy to "relate" to characters that really don't have a major role in the story after the first 3-4 hours of the game. After that, they are just sightseers along for the ride. Had the game been Balthier/Fran/Ashe/Basch in a four man party with no Penelo/Vaan, and they somehow tied the Occuria into the story more (and made -them- the last boss) as well as having Vayne do some more sinister shit throughout the game to really put him in "bad guy" territory, it would probably be one of the best RPGs ever made. Instead it is just really good with some story problems.