I generally find that 5e has a good balance of rules and freedom, possibly getting close to optimal as far as a D&D chassis goes. To me, it feels like how I thought D&D should play back when I first started and all the inconsistencies and nitpicky details in AD&D bugged me, even though I loved the game. In practice, this means that there is more weight given to "second-order design," so that the game feels cooperative - the DM and players are working together. That said, I really want to try Dungeon World, because having read the rules it potentially offers an even more satisfying balance of shared storytelling.
However, one thing I note is that 5e is still a pretty granular game when it comes to combat. In fact, the rules are easily detailed enough that it can still be played much like OD&D: as a miniatures-based war-game. I rather think that 5e intentionally went back to more of an OD&D philosophy in that regard, striving to free up DM and player cooperative storytelling outside of combat while keeping tactical play quite tightly constrained. I'm a miniatures and terrain enthusiast, so for me this is a huge plus (I am resisting the urge to post a photo of the battle map from our last game).
For me, the D&D rules scratch three itches, in fact:
1. They are constrained enough to run a skirmish-level wargame - first order.
2. They are open-ended enough to work decently with cooperative roleplay (though I think other systems are better) - second order.
3. They are enjoyable to read and play around with in their own right - ???
I don't think there has been much discussion yet of 3, but I strongly feel that for a certain type of gamer, like me, there is huge enjoyment in just reading rules and thinking about the game. Maybe rolling up different characters, or imagining the kinds of stories I could run. I don't think this falls into either the first or second order category, but is rather a marriage of an aesthetic and logical experience. And enjoyment of rules for their own sake is widespread - who here has not backed a Kickstarter or bought a rule book mostly just to read through and perhaps find inspiration, even knowing full well that you might not ever play the game?
I wanted to get back to this point. I once wrote a post about a similar topic-
You know what people love? BIG WORDS! So I thought I'd put a bunch of 'em in the title. I mean- those aren't just your nickel or dime words. That's a Sacagawea! Anyway, I wanted to get an issue of off my chest I've been thinking about for a while. It's closely related to something I wrote about...
In effect, the success of D&D as a mass market game is because it is not just a product of rules, but a comingling of the rules, norms, community, and history. In effect, the success of D&D is that it implicitly allows for a multitude of gameplay; that it does not speak to (or constrain) second-order design, and, moreover, explicitly allows a diversity of play in fundamental ways, even to the level of whether you play ToTM or grid.
That said, the question of aesthetic enjoyment of gaming materials is also an interesting one. The art, the lore, the dice, the minis .... those are all things that people can appreciate even if they aren't currently playing the game.
But I am curious about your last point. While I think that I've enjoyed the lore of RPGs, I am struggling to think of ways in which I've appreciated the rules of an RPG for their own sake. I've certainly been intrigued by different rules, but I don't know that I would ever buy a TTRPG just for an aesthetic appreciation of the rules qua rules. Did you have something specific in mind?