I think characterization is the reason why Kobayashi's works tend to be seen as so good. Characters are hardly one-dimensional, and tend to be far deeper than they look at first glance. A good example, for me at least, is Eiji: at first he looks incredibly bland, being just a nice guy who helps people (something Kobayashi absolutely loves), but then we're shown that his attitude has reasons behind it, and it's not seen as good. Rather, he's shown as a broken man.
Her characterizations work really well in the world they're in, as well. Shinji, for instance, is basically what you see at first glance and nothing more: a nice guy who helps people. However, Ryuki is extremely cynical on this, and even Shinji ends up wondering if he's doing the right thing. It makes the audience wonder it, as well, is he right in stopping the rest of the Riders from fulfilling their dreams, just to stop the war? The ending ended up being extremely anticlimactic, but I like to believe that was forced by Toei since a complete sad ending (every Rider dies) would not be liked by society or something, I dunno. I just like to forget that last part ever happened.
On the other hand, Kobayashi's series always seem to have the same problem. Great beginning, great ending, awful middle. Shinkenger is probably my favourite Sentai, and I find the middle part almost unbearably dull. Go-Busters, surprisingly, doesn't feel like that, though that's probably because of the whole Enter business.
Talking about Enter, another thing I, at least, greatly enjoy of Kobayashi's works is how the villains are hardly typical, and hardly bland. Kanzaki, Enter, the Greed and Maki, all of them are explored, their reasons are given, and they all are not only interesting as characters, but dangerous as villains. Sentai is, most of the time, incredibly bad at the latter, we see the heroes win, the villains barely manage anything, and so, any tension is immediatly lost. This is why I consider Enter the best Sentai villain I've seen: He WINS. Often, at that. Even when he loses, he achieves something. The last part of the series ends up feeling stronger just because of how Enter is a lot more important, going from the Dragon to the Big Bad. Even moreso in the last few episodes.
There's exceptions, of course, like the Imagin (and whatever was his name, Kai?), who...aren't developed, aren't interesting, and are horrible villains and even worse heroes, but I admit I'm biased on this one since I really don't like Den-O.