FF9 has often been touted as "getting back to the series' roots". I've played FF since FF1 on the NES, but for me this game just didn't capture the feel of FF1-6 at all. I'm not sure exactly what it is, but I mostly attribute it to two related problems: an excessively cartoony graphical style, and a plot with far too many "lighthearted" and generally not serious elements in it.
You're disappointed because it's not just one big amalgam of previous games? Sorry, but not even those games as standalones represent everything the series has to offer. FFIX is supposed to be eclectic, cherry-picking the best from previous games. I wouldn't say the artistic stylings are 'excessively cartoony' either -- it's a shakeup from the overly-humanistic FF7/8 worlds where many FF games had most of the 'fantastical' elements squeezed out of 'em and replaced with a bit o' magic, a summon, etc. FFIX goes the whole nine yards in delivering the 'fantasy' that the series namesake touts, and I for one am very glad they did. 'Course, if you're not even sure why you don't like it, that's that.

FF9's graphics are distinctly cartoony, while FF1-6's generally were not (granted they each had their own individual style) except for the SD character and townspeople sprites. FF6, for example, had excellent and mostly quite serious monster art that still holds its own today. The same goes for the dungeon and town art. Compare these with FF9's and the difference is immediate. FF9 used a cartoonish style for the dungeons, towns, FMV, and character portraits (again, compare these to those from FF2, 4, and 6). Alexandria reminds me of something from "The Wizard of Oz", not FF1-6, and Brahne's ridiculous design has no precedent anywhere else in the series (compare her to the other "initial villains", such Garland, Golbez, or Gestahl (sp) for example) . Even FF1 often seems more serious, with any cartooniness coming more from a limited art budget and NES graphical limitations than anything else.
You're comparing games with low GFX capabilities to a modern-age game -- naturally there is going to be disparaties. I wish I could compare Brahne to previous baddies, but then I think, "Gestahl was five pixels wide and Brahne has CGI sequences built around her."

FF9's plot and characters also have a non-serious tone about them that, again, was totally new for the series up to that point, with the partial exception of FF5. I'm aware that there are some very serious moments in the game, especially towards the end, but the the whole setup with Queen Brahne (complete with totally absurd appearance) and her army of pointy-hatted, yellow eyed black mages seems like it belongs in a Disney movie. Steiner and Quina's character development and designs are also out of place for the series. Quina is the only overt joke character I can think of, and Steiner's "bumbling prejudiced fool" personality becomes tiring and annoying, and it also turns him into a borderline joke character.
See, it's stuff like this that doesn't make sense to me. Steiner is portrayed as an Alexandrian zealot, overly protective, has a strong sense of duty, doesn't want to be involved in wrongdoings, etcetera...and then he gets a complete one-eighty later on, choosing to rely on Zidane, the thief who abducted his princess. He even goes so far as to say something like "I haven't decided if you're the right man for the princess!" Naturally three discs' worth of development can't be summarized in one post, but there's no sudden OOC change that occurs -- he's pretty well-rounded when it comes down to it. Quina, on the other hand, isn't supposed to be anyone's foil and is, in essence, single-minded (like whoa).

Just as you say that things are "Disney"-like, I have to wonder: there's nothing wrong with the animation, the black mages aren't shown as happy-go-lucky things that want to play in flowerbeds, Brahne's never portrayed in an overtly comical moment. Since dialogue really pushes the game forward, and that can definitely be less-than-serious, I think that might be part of the problem. 'Course, I don't watch anything Disney so I'm not really up to speed on that sort of thing (KH is foreign, yeah), but the game has a distinctly dark tone for quite awhile -- a fat elephant-lady and some mages that are shown in that respect doesn't equate to Disneyesque things, to me.

I'll probably get flamed for this, but all in all the game ends up feeling more like "Final Fantasy Mystic Quest done right" than anything that recaptures the feel of FF1-6. It's ironic that FF12, a game that's a radical departure in many ways, actually succeeds more in capturing the "epic sweep" of the earlier games than FF9.
FFXII has a more world-centric point of view than character-centric, so while you could say that the "sweep" was done better, I'd have to say that FFIX didn't attempt to be epic. It takes four discs, yeah, but it doesn't have that air of trying to be something it's not, overexerting itself and falling drastically short. I played XII, wrote a guide for it, debated plot points...I wouldn't call that game better than anything. Characters are just 'vessels' for the player to live through; FFXII is very, very hollow in that respect, using video sequences to advance the story as opposed to intrapersonal dialogue. Funny how people perceive radical changes, huh?

Naturally, yer entitled to whatever you want to think. At least you didn't say you hate the game because the main character has a tail.