I like the variety in the Final Fantasy series. Really, if things stayed static, the games would start to get boring. There has to be innovation, and there has been innovation. We'll stick merely with the gameplay modifications.

Final Fantasy II was a drastic step away from the original in many ways. It attempted a new leveling system (although they didn't balance it properly until the GBA port).

Final Fantasy III introduced the new class system. I've not yet played it, so I can't comment much more on that.

Final Fantasy IV was the first to use an assigned class system.

Final Fantasy V went back to a class system similar to that of FFIII.

Final Fantasy VI made use of espers to give all characters access to spells and summons while seperating characters into semblances of classes similar to that of FFIV through individual abilities.

Final Fantasy VII used materia.

Final Fantasy VIII used the junctioning system.

Final Fantasy IX was a throwback to the class system, but it used equipment to teach abilities.

Final Fantasy X used the sphere grid.

Final Fantasy X-2 used the class system with a spin through dress spheres.

Final Fantasy XI implimented the class system in a manner very similar to that of Final Fantasy Tactics.

Final Fantasy XII used the ADB system.

The only games that I can't acknowledge the drastic differences in are III and V, but that's 'cause I've not played III at all and very little of V.

It's only natural that the games will continue to change. Innovation is necessary to keep both the series and genre alive. As for complains about spin-offs, I supposed I can agree to that. All the Final Fantasy VII games are starting to make me sick (the only one I was ever really interested in, and the only one that looks like it'll be good is Crisis Core). Otherwise, un-numbered Final Fantasy games usually just use the name to sell. I know the Final Fantasy Legend series has a different name in Japan (although I cannot remember which it is).

By the way, Final Fantasy Tactics is freaking amazing. Leave it be.