There are a lot of players who pretty much auto-plan their feat tree as part of their concept. They want their character to the best they can be at the thing they are designed to be good at. I don't have a problem with that - you use the tools you have at hand and those players are approaching their character mechanically. However, you can also approach the character from the other end.
I don't know what you mean by
approach the character. Do you mean
think about the character's motivations? Do you mean
think about how the character will achieve things as part of the action resolution process? Something else?
@DEFCON 1 seems to be asserting that,
in 5e D&D, a free descriptor can be the main component of action resolution. If that claim was being made about HeroQuest revised, or about Fate, or even about Cortex+ Heroic, it might be plausible. Though even there there are limits - a player who builds a HQ revised character with the descriptor
Best swordsman in the land but who puts a low rather than a high rating against it doesn't seem fully committed to the claim on behalf of his/her PC, and so one might expect some irony or self-realisation to result from play..
let's say the character's belief is "I am a deadly swordsman and I will prove myself the best in the land". That's a really cool character belief (like Burning Wheel good). I completely agree with DEFCON's take on how that belief works from a narrative perspective at the table as well. You don't need any mechanical support to do things this way. but that wasn't the point - the point was that the narrative side, the character belief side, is (or can be) a much more potent way of realizing that concept at the table. Obviously you need some mechanics in place, and good decisions about character build will help, but it doesn't need to be optimized.
But if I build a BW character with that Belief, who has a 3 Agility and Speed and no ranks in sword, then my character is unlikely to actually realise that Belief and earn a persona for it in that particular fashion.
I might earn Fate from that Belief with that character easily enough; and I might earn Persona from Embodiment or Mouldbreaker in which that Belief figures. But the BW rulebook itself has stipulated ranks for expertise, and tells us that an
expert typicaly has exponent 5 ((I'm on p 15 of Revised, but the chart in Gold is the same). So my Exp 0, Beginner's Luck sword fighter is objectively a deadly swordsman in the most literal sense of that phrase and hence is unlikely to present as such in play.
An interesting feature of BW play, which Luke discusses in some detail in the Adventure Burner/Codex commentary, is that play can reveal a Belief to have a non-literal, non-anticipated or even ironic meaning. And that could happen here - my
deadliness might turn out to be to myself, or my allies, or to have some other unexpected meaning in play. But I don't think that many D&D players are looking for that sort of experience, and I don't think that's what DEFCON 1 had in mind either.
the character's beliefs are not set by the mechanics, not should they be, nor should the players beliefs about the character be bound by the mechanics. All we can do with the mechanics is get a close as possible to our concept, everything else is down to narrative.
The player of a D&D PC is free to play the PC with the belief
I am the greatest magician in the land, but I think there are very few D&D tables at which that belief can be realised if the PC is (in mechanical build) a low level champion fighter with no spell slots or other magical abilities.
D&D has rather concrete mechanical elements of PC building that are connected to concept. This is most obvious in the magic rules, but is also a feature of other aspects of PC build. For instance, kobolds are set out in the MM as having certain attack and damage numbers; and nearly every D&D player knows that kobolds are at the bottom of the combat capability food chain. So a PC whose combat stats are no better than a kobold's is, ipso facto, not really getting very close mechanically to the concept of
best swordsman in the land or even
deadly swordsman.
Many players (like you apparently) fall into the group that equates game mechanics with ability.
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I, however, find that way of thinking needlessly restrictive, because it means that any character you start with at like 1st or 3rd level , is by definition a piece of garbage
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Which, granted, is how WotC kind of defines the Levels 1-4 tier so it does have its place... but personally I think that's a stupid way of looking at it.
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So I just get around that whole thing by not equating game mechanics to the fiction of the world. You can be a well-known swordsman even at 1st level in the narrative, because narrative doesn't care about mechanics. It is what someone does in the story (before, during and after the campaign) that determines how good they are, and how well-known they are, and how well-respected. If you make a PC and define them in the story as a 10 year veteran in the military, then that PC has the status, knowledge, and skill of a 10 year veteran and gets treated as such, regardless of whatever level they start at for purely game purposes.
When
my group started its Prince Valiant campaign, one of the PCs was a 40 year old knight who (as per the player's description)
had accomplished little. When the same group started a 4e D&D campaign several years earlier, one of the PCs (played by a different player) was a 40-something year old 1st level wizard.
But the mechanics of Prince Valiant - not to mention the experience of playing that first session - made it clear that the PCs were not (for instance) the equal of Sir Lancelot, and indeed were not very strong knights. And the interaction of 4e mechanics and fiction (our campaign mostly just followed the default for this set out in the various Monster Manuals) made it clear that the 1st level PCs, while more puissant than a typical non-entity NPC, were not as potent as (say) the magician leader of the evil organisation they were opposing.
Your post posits some sort of contrast between
fiction of the world and
narrative, on the one hand, and
game mechanics/game purposes on the other. I don't understand what the contrast is meant to be, because I don't see how the fiction and narrative are independent of the outcomes of action resolution which (typically) are determined by application of the action resolution mechanics. This is what @AbdulAlhazed is getting at when he asks about the self-described
best swordsman in the land being easily taken down by a town guard (or typical orc warrior, or whatever).
4e is the version of D&D that went furthest in allowing a player's conception of his/her PC to be realised without worrying about how (say) a typical town guard or typical orc thug might prove it wrong, because of its tendency not to stat up non-entities, its use of minion rules, its clearly articulated tiers of play and the associated fiction, and other well-known though widely despised features. 5e has less of all this, and is more insistent in its design that a certain mechanical element (eg +2 to hit, 15 AC, etc) correlates to something rather concrete in the fiction.
And 5e is also much more concrete than 4e in respect of magic. To me it is rather striking that you set out your thesis by reference to
the best swordsman in the land rather than
the best wizard in the land or
the best planeswalker in the land.