When people say that IX went back to Final Fantasy roots, I agree with it partially because VII and VIII were very much different and were very futuristic when compared with I to VI.
When FFIX came along, it was much more like I through to VI than it was to VII and VIII if you catch my drift. If I had to categorise them, I'd put them something like this:
I, II, III, IV, V, VI and IX.
Then...
VII, VIII, X, X-2 and now XII - (Which are more futuristic in their worlds).
I'll sum them up from what I know about them.
FFI - I don't know much about. Haven't played it much, but it was the first of its kind, and therefore true to itself. One fine point is that is has four "heroes".
FFII - I've not played much of this one either, but it's almost identical to I in the way it works. And again, it has four heroes - but this time they have some kind of personality to them and already have names.
FFIII - Only just started to play it on the DS. Never had a chance to play the original. Still, it feels completely traditional to its predecessors. And of course for the third time running, four heroes (that are named like in FFII).
FFIV - I've played most of it, and it feels like a true hero fantasy story to me. You know, Cecil and Rosa, and then the powerful evil Golbez. Again, traditional.
FFV - I would brand it the same as FFIV, though there are four true heroes which reverts back to how FFs I, II and III worked.
FFVI - It felt like it was set in a time like the 1600s-1700s. A sort of Industrial Revolution kind of setting. It was much different to its five predecessors, and had many more playable characters, each with an inspiring personality. It was still done in the traditional FF style to date.
FFVII - Ah, now we're seeing change. The world is starting to become a bit more 3D, although the environments were pre-rendered. It's not top down anymore to say the least. It sells more copies than any other FF game, and it's world is set in a dark, very industrial future. It still copied most ideals from the earlier games. The difference was the very dark industrial setting.
FFVIII - Suddenly everything goes futuristic again, but in what seems to be a much brighter world. You can't really see evidence of pollution, and the world itself appears to be quite content in the long run. Buildings look bright and colourful. Balamb garden first appeared to me to look like a futuristic church, and the bells sound like there is a wedding going on. This game didn't feel quite so Final Fantasy like as the others to me. Maybe the graphics and drastic change in the world gave that impression?
FFIX - Now we're getting to the point. Ask yourself how much this game has in common with VII and VIII. In my opinion, very little. When I compare it with every game before VII, it feels like its world has returned to the way the others worked. The world was much more medieval/renaissance, and the airships served a much wider purpose. I like VIII, don't get me wrong, but flying schools fighting one another just didn't really do it for me. That's what I feel is meant by going back to Final Fantasy roots.
I can't be bothered to go over the others really. X and X-2 felt similar to VIII in terms of the bright and colourful world, and XII feels like a graphically up to date version of Vagrant Story rather than a Final Fantasy game. In addition to that, the airship scenes and imperial city of Archades reek of Star Wars no?