Well, it was great series until they decided that the story was too linear. They decided screwing with everything was the way to go.
Personally I think the general direction of the series has brought it down. There was no point in continuing Sora, Riku, and Kairi's story past KH2. The beauty of the first KH game and by extension KH2 was that they used the source material from Disney so well, in fact they pretty much exhausted Disney's very best stuff within those games. I've noticed that the more focus on the Disney stuff, the better the games were. Conversely the more 'Nomura' stuff there is the further the game tends to go down hill.
KH1 was great because it was a simple and easy to follow story that didn't need a lot of explanation. The game didn't have to build up the villains for us because they were villains everyone grew up with. We knew them and we knew what they were about. Nowadays however it's all about Xehanort and darkness and....an extremely convoluted and unnecessarily complicated story that has been told in an absolutely horrible way. In fact, I remember the tagline of the ads of the very first KH game back in middle school, 'You'll never know who you'll run in to next'. That was the pull of the game, the focus on the Disney.
Personally I think Kingdom Hearts as a series would have gone so much better if they would have gone on to different franchises. For example, after Kingdom Hearts they leave Sora, Riku, Kairi and all of Disney alone and go on to something else because that story was done and the Disney stuff had been played out to its fullest. I mean, the only real key components of Kingdom Hearts are the Keyblade, Heartless, and multiple worlds. Those could be put in to so many different types of scenarios. Imagine if they would have done another Kingdom Hearts trilogy with another group of original characters and went to the worlds of Final Fantasy or perhaps Pixar? It'd probably a lot better than the mess the series has become. (FYI, this is all coming from someone who was absolutely enthralled with the series back in the day. I'm just sad to see what it's become.)