I don't think there's anything "plain" or "simple" about KH1. It is very layered in its philosophical approach and you can walk away with several different interpretations on what you've witnessed and experienced throughout the journey. On its face, it's a coming of age story that has a typical good vs. evil plot. But if you delve below the superficialities of it, you can come away with several unstated conclusions.
For example, after playing the first game a few times, I walked away with the impression that Kairi is very manipulative. Considering she has total free reign within Sora's heart, it's not outside the realm of possibility that she's responsible for guiding him to the ultimate destination of Hollow Bastion. It all starts at the docks when Kairi remarks "Riku's changed" and then says, "Sora, let's take the raft! Just the two of us!" She clearly wants to go home and clearly doesn't want Riku come along. When she lost her heart, she buddied up with Sora and not Riku. At first glance, you're led to believe that this is simply an act of timing and necessity, but upon further reflection, you can postulate that she didn't want to be with Riku.
Both Riku's and Kairi's boats were docked at the island long before Sora's. It's possible that Riku reached the island first, opened the door, and then stood in wait for the ultimate crossover. Kairi tried to close the door and failed. She didn't want to go with Riku. She wasn't standing in wait with him. She wasn't even arguing with him. Kairi ditched Riku and ran for the door. We don't even know if she was physically standing at the door at that point. Maybe her heart was fluttering around, but not her physical shell. We know that hearts as well as Heartless can take human form, so we can't altogether conclude with 100% certainty that Kairi was physically standing at the door when Sora rushed into the Secret Place. If you take this holistically--Kairi wanting to leave with just Sora, her avoiding Riku, and her final acts before losing her heart--you can plausibly argue that Kairi's ultimate goal was to go on adventure with just Sora and to find her way home with just Sora. This doesn't mean that she doesn't like Riku or that she's not friends with Riku. It just means that in this instance, she was deterred from dealing with Riku. You can break this down into two categories: (1) wanting to spend alone-time with Sora or (2) wanting to avoid a threat she perceived within Riku. Why she would perceive a threat from Riku is arguable, but she definitely sensed something when she remarked that "Riku's changed."
After the DI scene, we know Kairi is piggybacking off Sora. Along the trip, she makes comments to Sora, appears before him, wanders off where he follows. She's the reason he strikes a bargain with Tarzan; Sora originally wanted to inquire after Donald and Goofy, but when he saw Kairi, he changed his mind. You can argue that this is a subtle manipulation. In the tent with Jane looking at the slides, Sora "recognizes" the castle. This is an indicator on Kairi's part. She wants to go home. Much later, when Sora begins to lose heart, she shows him the memory of her grandmother inside the castle. This flashback is instrumental to Sora's ultimate success later on when confronting both Riku (after the Keyblade was "stolen") and Ansem (when in front of The Door). Without that revelation given to him by Kairi--who didn't know whether she would get her body back and was therefore trying to manipulate Sora from within--Sora would not have been successful in stopping Ansem.
Granted, you could easily counter-argue that Kairi was just along for the ride and wasn't trying to intentionally influence Sora. But I think there's a lot of evidence to suggest that every time she intervened, it was a conscious effort to manipulate Sora into doing what she wanted. This makes it sound like a bad thing, but not all manipulation is inherently "bad" or "evil." In this case, she was just trying to guide her champion to success and to her ultimate goal: adventuring towards home. At least that's what I think.
In any case, I don't think there's anything "simple" or "straight to the point" about that lol.
At any rate, I do agree with you. I think KH1 is the best in the series. It's a neat package without any seriously damaging loose-ends. The biggest loose-end is trying to rescue Riku and Mickey, but that's more cliffhanger than incomplete plot point. I don't like how convoluted the series has become with the introduction of the multiple Xehanorts and the Organization. But what bugs me the most is how the Heartless have become a laughing stock. I really liked 2.8 because Aqua's story shows the Heartless to be pretty fearsome. As a kid playing KH and seeing Darkside for the first time, I was awestruck. That's what an evil entity should be. The Shadows, the Neoshadows, and Darkside--the trifecta of evil. They're creepy to look at. But nowadays, the Heartless seem to be the butt-end of every joke. They're barely even mentioned or present anymore. I think KH1 struck a good balance of terror and hope. Later titles are a zanier, though BBS I think is a good spiritual successor to KH1. It seems to capture a similar essence.