I have never spent 3 hours coding, so I can't comment on that.
I also don't think my RPGing has an "empty calories" quality, so I can't really comment on that either.
On Thursday afternoon I spent about two hours with a colleague working on a paper that we are authoring together. We wrote about two pages; but did a lot of other things too, like confirming some patterns across a series of statutes; and also identifying, mis-characterising, and then correcting our characterisation of, an unusual feature of another statute sometimes treated as belonging to the same series; and developing and outlining some further expository and analytical "moves" that we will make in the paper. That time passed much more painlessly than sitting in a typical two hour committee meeting.
When I say that the play of an RPG can be deeply involving, focusing activity and attention in a manner that creates a degree of oblivion (or at least modest failure to pay attention to) other stuff going on around me, I have something broadly similar in mind. I wouldn't describe it as "soaking in novelty" because I am not solely or probably even primarily an audience member. It's a collective, creative activity: there's the expression of one's own ideas, the riffing on the ideas of others, guided by systems and (on some occasions) by prep.
I have taken part in RPGing which isn't immersive in this sense; where is it more like a social gathering, with the game as a secondary concern or a socially facilitative activity. I've also taken part in RPGing which aspires to be like this, but where certain techniques being used disrupt the creative interaction. Sometimes Rolemaster could be like this, for instance, because of the way certain of its mechanics work.
Immersion in the fiction of the game is different from immersion in the activity of playing a RPG, but I think that, for me at least, the latter is a necessary condition of the former. (And immersion in character is, I think, a special case of immersion in the fiction.)
I don't dislike rationed abilities. But - speaking for myself, and my experience as a player of RPGs - I find that they can be at odds with immersion in the fiction, at least in the context of a broader set of meta-concerns like "when will the GM let us rest" or "what else would the GM have put into this adventure". Again, I don't dislike that sort of RPGing; I just think it doesn't fully conduce to immersion in the fiction.
The RPG I mentioned in my post as the one I find the best for immersing in the fiction as a player - Burning Wheel - does have limited-use abilities, namely, Fate and Persona points. But because the basic function of these is to let you try harder (by adding to your dice pool), deciding whether or not to use them doesn't - at least for me - disrupt my immersion in the fiction of my character. Quite the opposite - it leads me back into my character, and understanding what my character is driven by, and how hard they are willing to try. A further aspect to this is that, if a character suffers a prima facie mortal wound, they survive only if they have The Will to Live, which requires spending a point of Persona. So choosing to spend your last persona point on something else is making a very strong expression, as one's character, that one is willing to die for <whatever is being aimed at>.
Addressing some of Swarmkeeper's questions: I personally find some typical ways of using patrons and adventuring organisations quite lampshade-y: that is, it's pretty obvious to everyone at the table that the real reason for the players agreeing to have their PCs take up <this particular challenge> is that it is the one the GM has in mind, has prepped, etc; and the patron (or similar) is really just hanging a lampshade over that. What makes Burning Wheel immersive, for me, is that it doesn't use a GM-preps-adventure approach to framing scenes and presenting obstacles. Everything starts with the characters. So there's never anything "meta" about why my PC is doing, and cares about, <this thing>.
Prince Valiant, which I also mentioned, does use prepped scenarios. But the PCs are knights errant (or part of their entourage), and the scenarios speak pretty directly to the concerns of errantry. And there are system features (its simplicity, but also the way it allows conflicts to be resolved and framed) that make it very easy, as a scenario is unfolding, to bring to the fore the concerns the players have established for their PCs (which, by the nature of the game, will have some relation to Arthurian or similar tropes and themes). And to make this a little more concrete in practical terms, a Prince Valiant scenario of 2 pages will generally support an hour or two of play (depending on the details of those pages!) - so when I'm talking about simple mechanics and the idea of building on the tropes and themes and presented conflicts to further draw in the players, and support everyone's engagement with the shared fiction, it's not like doing that in (say) a D&D dungeon module.
