Maxperson
Morkus from Orkus
It starts to be noticeable at +3 or +4 difference. Someone with a 20 will succeed quite a bit more often than someone with a 12. But with 5e's low DCs and ACs, a 20 is a waste of time and effort to achieve. The benefit of a 20 over an 18 is miniscule and a feat is much more impactful.Ok, but what argument are you making, then? Are you saying that the numbers don’t matter? Is there a magnitude where they start to matter? Are they meant purely as an aid to characterization?
The core issue with the “balance doesn’t mean much” idea is that it means all the progression and decision making we do around our characters isn’t really that important. And that means we’ve moved from “playing a game” to “engaging in a shared activity”.
I actually agree with you that in the D&D case the numbers aren’t that meaningful, which is why I’ve mostly moved to other games with more meaningful decision making.
The only time my stats increase after I make a PC is if the feat I want just happens to come with a +1 or if I get a bump from the DM side of things. Picking ASI's instead of feats is gimping yourself.

