I don't think discovery is always secondary to the internal world of the character, I have an actor player or two, but I also have players much more focused on everything else.
What is the point of RPGing? I think for many if not most people it's not to learn the abstract or impersonal details of an imaginary place. It's to enjoy the exploits and struggles of one's PC.
Exploits and struggles are not confined to the PC's internal life. I think in most RPGing, the internal life of the PC is not all that vivid - it's the external life - the fighting, the sneaking, the looting - that tends to be the focus of play.
But this is nevertheless quite different form learning the abstract or impersonal details of an imaginary place.
I don't think that's what discovery means to most people. I think it is more about exploration and surprise, and it's probably not especially important if what they are discovering is deep lore or intriguing ephemera or the friends they made along the way.
Sure, I'm not saying it's the whole of Discovery, but I do think it can be a meaningful component. But I can obviously only speak directly for myself.
I think that the idea of "discovery" as it relates to the fictional world is best if used sparingly. Players in an RPG are part, but not fully, acting as the audience. And an audience may like to learn things along the way. But is that suitable for play?
If as a player you are trying to learn something through the vehicle of your PC - the classic example is a PC exploring a dungeon, but obviously there are indefinitely many other possibilities - then discovery can be exciting. But as
@Reynard says, I don't think it normally matters whether what is discovered is profound or relatively shallow.
In my most recent Torchbearer session, the players via their PCs discovered a throne that lets the one who sits in it project their point of sight out into the world, "flying" about to spy on the landscape. This was literally a discovery, in that they (and their PCs) did not know about this throne until they found it and then experimented with it (during the experimentation one of the players did conjecture its purpose, based on some features of its layout and the room it was in).
This sort of thing can be exciting discovery in a RPG.
But it doesn't depend on
lore at all. In the fiction, the throne has a creator, but that creator has a name and two lines of backstory (most of which the players have learned, by having their PCs undertake research). If more backstory is needed of course it can be authored, but that will be in further relationship to what the PCs are doing. I'm not looking for a chance to just drop in some lore that does not bear upon or follow from the players' play of their PCs!
I have GMed games that have been regarded as "lore rich" and "lore driven" by those who play in them and by those who observe them. What I learning GMing those games was that lore works well as a context or framework
for play. But it is the players' play of their PCs which is key. This also informs what counts as "good" lore: it should be relevant to play, and potentially be driving of play. This is why I think 4e's default lore is great - it is laden with conflicts that players can easily by into just through building their PCs - whereas, say, genealogy that is not relevant to establishing conflict or driving action (eg JRRT's genealogies of the Kings of Rohan and the Ruling Stewards of Gondor) is in my view pretty optional.
