I rather enjoy Kingdom Hearts. Here's my opinion for those that don't like it, or think the game is spread out across too many consoles...
It's an action RPG. So no, it's not turn based, and yes, you can spam the action/attack key pretty much the entire time and still beat the game on normal difficulty. But then again, I did the same thing in God of War. Now take a heavier difficulty level, and you're going to need to use some strategy. And probably figure out how to use magic. It's not turn based.
It has 3 different franchises rolled into one. Yes, the entire game is fan service. C'mon Final Fantasy characters and Disney characters? I said three franchises. Sora, Roxas, and Org 13 are now big enough to be a franchise on their own. Does this get muddled?
Kingdom Hearts 1 was literally the saga of Sora finding his friend Riku and his adventure along the way... Without spoiling it, he finds him but the journey isn't over, hence Kingdom Hearts 2. So you can pretty much get the entire feel for the series with those two games. Just own a PS2. You want to fill in the blanks? Because some events definitely occurred between 1 and 2 (which set up 2)...
Rent ReChain of Memories, the PS2 remake of the GBA game. It uses the same engine as the PS2 games, but it still retains the card mechanic, with an action key... It's not for everyone, but hey, it'll let you know about the rest of the story.
358/2 Days is a side-story about Organization 13 (a big plot point in KH2) and one of the main characters Roxas. It was clear it was retconned into the series, so it's not a NEED to have game. It's timeline is basically the year before KH2's events.
ReCoded is a side-story about the timeframe directly after the events of KH2. Again, it's not a need to have, just like the other 2 aforementioned games.
Birth By Sleep is the prequel to Kingdom Hearts, and there are a lot of hints dropped that it will set up some of the events of KH3 (I've avoided reading any plot summaries for the Japanese version), it too will probably be avoidable.
So yeah, to play the core series, just play KH1 then KH2. If you are feeling extra adventurous, you can strike out for Re:Chain of Memories in between the two. And if you really like the gameplay, storyline, and characters, you can get the NDS games to fill in the gaps.
for those TL;DR people:
You only need to play the 3 Playstation 2 games: Kingdom Hearts -> Kingdom Hearts - Re:Chain of Memories -> Kingdom Hearts 2